[DESCRIPTION] [Title of Invention] IMAGE CODING METHOD, IMAGE DECODING METHOD, IMAGE CODING APPARATUS, AND IMAGE DECODING APPARATUS [Technical Field] [0001] The present invention relates to an image coding method of coding each of blocks of pictures. [Background Art] [0002] A technique relating to an image coding method of coding each of blocks of pictures is described in Non Patent Literature (NPL) 1. [Citation List] [Non Patent Literature] [0003] [NPL 1] ISO/IEC 14496-10 - MPEG-4 Part 10, Advanced Video Coding [Summary of Invention] [Technical Problem] [0004] However, there are cases where the conventional image coding method cannot achieve sufficiently high coding efficiency. [0005] In view of this, the present invention provides an image coding method that can improve coding efficiency in image coding. [Solution to Problem] [0006] An image coding method according to an aspect of the present invention an image coding method of coding each of blocks of pictures, the image coding method including: deriving a candidate for ajmotion vector of a current block to be coded, from a motion vector of a co-located block which is a block included in a picture different from a picture that includes the current block; adding the derived candidate to a list; selecting the motion vector of the current block fronvthe list to which the candidate is added; and coding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the co-located block is a long-term reference picture or a shoirt-term reference picture; deriving the candidate from the motion vector of the co-located block by a first derivation scheme that does not;involve scaling based on a temporal distance, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate from the motion vector of the co-locate:d block r by a second derivation scheme that involves scaling baseb on a temporal distance, in the case of determining that each! of the : reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0007] These general and specific aspects may be implemented; using a system, an apparatus, an integrated circuit, a computer program, or a non-transitory computer-readable recording medium such as a CD-ROM, or any combination of systems, apparatuses, methods, integrated circuits, computer programs, and recording media.: [Advantageous Effects of Invention] [0008] The image coding method according to the present invention provides an image coding method that can improve coding efficiency in image coding. [Brief Description of Drawings] [0009] [FIG. 1] FIG. 1 is a flowchart showing an operation of an image coding apparatus according to a reference example. [FIG. 2] FIG. 2 is a flowchart showing an operation of an image decoding apparatus according to the reference example. [FIG. 3] FIG. 3 is a flowchart showing details of a derivation process according to the reference example. [FIG. 4] FIG. 4 is a diagram for explaining a co-located block according . : ' to the reference example. [FIG. 5] FIG. 5 is a block diagram of an image coding apparatus according to Embodiment 1. [FIG. 6] FIG. 6 is a block diagram of an image decoding apparatus according to Embodiment 1. [FIG. 7] FIG. 7 is a flowchart showing an operation of the images coding apparatus according to Embodiment 1. [FIG. 8] FIG. 8 is a flowchart showing an operation of thej image decoding apparatus according to Embodiment 1. [FIG. 9] FIG. 9 is a flowchart showing details of a derivation process according to Embodiment 1. [FIG. 10] FIG. 10 is a flowchart showing details of a derivation process according to Embodiments. [FIG. 11] FIG. 11 is a diagram for explaining a co-located block according to Embodiment 2. [FIG. 12] FIG. 12 is a flowchart showing details of a derivation process according to Embodiment 3. [FIG. 13A] FIG. 13A is a block diagram of an image coding apparatus according to Embodiment 4. [FIG. 13B] FIG. 13B is a flowchart showing an operation of the: image coding apparatus according to Embodiment 4. [FIG. 14A] FIG. 14A is a block diagram of an image decoding apparatus according to Embodiment 4. [FIG. 14B] FIG. 14B is a flowchart showing an operation of the image decoding apparatus according to Embodiment 4. [FIG. 15A] FIG. 15A is a diagram showing a first example of a storage location of a parameter indicating a reference picture classification. [FIG. 15B] FIG. 15B is a diagram showing a second example of the storage location of the parameter indicating the reference \picture classification. [FIG. 15C] FIG. 15C is a diagram showing a third example of the storage location of the parameter indicating the reference ipicture classification. [FIG. 16] FIG. 16 is a diagram showing an example of a storage location of a parameter indicating a prediction mode. [FIG. 17] FIG. 17 illustrates an overall configuration of a content providing system for implementing content distribution servicjes. [FIG. 18] FIG. 18 illustrates an overall configuration of a digital broadcasting system. [FIG. 19] FIG. 19 is a block diagram illustrating an example; of a t configuration of a television. [FIG. 20] FIG. 20 is a block diagram illustrating an example of a configuration of an information reproducing/recording unit that reads and writes information from or on a recording medium that is an optical disk. [FIG. 21] FIG. 21 shows an example of a configuration of a recording medium that is an optical disk. [FIG. 22A] FIG. 22A shows an example of a cellular phone. [FIG. 22B] FIG. 22B shows an example of a configuration of the cellular phone. [FIG. 23] FIG. 23 shows a structure of multiplexed data. [FIG. 24] FIG. 24 schematically illustrates how each of streaims is multiplexed in multiplexed data. [FIG. 25] FIG. 25 illustrates how a video stream is stored in a stream of PES packets in more detail. [FIG. 26] FIG. 26 shows a structure of TS packets and source packets in the multiplexed data. [FIG. 27] FIG. 27 shows a data structure of a PMT. [FIG. 28] FIG. 28 shows an internal structure of multiplexed; data information. [FIG. 29] FIG. 29 shows an internal structure of stream attribute information. [FIG. 30] FIG. 30 shows steps for identifying video data. [FIG. 31] FIG. 31 is a block diagram illustrating an example of a configuration of an integrated circuit for implementing the moving picture coding method and the moving picture decoding miethod according to each of Embodiments. [FIG. 32] FIG. 32 shows a configuration for switching between driving frequencies. [FIG. 33] FIG. 33 shows steps for identifying video data and switching between driving frequencies. [FIG. 34] FIG. 34 shows an example of a look-up table in iwhich standards of video data are associated with the driving frequencies. [FIG. 35A] FIG. 35A shows an example of a configuration for shading a module of a signal processing unit. [FIG. 35B] FIG. 35B shows another example of a configuration for sharing a module of a signal processing unit. Description of Embodiments [0010] (Underlying Knowledge Forming Basis of the Present Invention) In relation to the image coding method disclosed I in the Background Art section, the inventors have found the following problem. Note that, in the following description, an image may be any of a moving image composed of a plurality of pictures, a still image ; composed of one picture, a part of a picture, and the like. [0011] Image coding schemes in recent years include MPEG-4 AVC/H.264 and HEVC (High Efficiency Video Coding). In these image coding schemes, inter prediction using coded reference pictures is available. [0012]* Moreover, in these image coding schemes, a reference picture ; called a long-term reference picture may be used. For example, in the case where a reference picture is retained in a DPB (Decoded Picture Buffer) for a long time, the reference picture may be used as a long-term reference picture. [0013] In HEVC, there is a mode called a merge mode. In the merge mode, a motion vector predictor obtained by predicting a motion vector of a current block from a motion vector of an adjacent block or the like is used for coding the current block as the motion vector of the current block. That is, in the merge mode, the motion: vector predictor is treated as the motion vector of the current block. The motion vector predictor and the motion vector of the current block in the merge mode are also referred to as a merge vector. [0014] In HEVC, a temporal motion vector predictor can be used, too. The temporal motion vector predictor is derived from a motion vector ; of a co-located block in a coded co-located picture. Coordinates of the co-located block in the co-located picture correspond to coordinates of the current block in the current picture to be coded. [0015] Hereafter, the motion vector of the co-located blockjis also referred to as a co-located.motion vector, and a reference picture of the co-located block is also referred to as a co-located reference picture. The co-located block is coded using the co-located jmotion vector and the co-located reference picture. Note that "co-located" may also be written as "collocated". [0016] Likewise, the motion vector of the current block is also referred to as a current motion vector, and a reference picture of the ;current block is also referred to as a current reference picture. The icurrent block is coded using the current motion vector and the icurrent reference picture. [0017] J The current block and the co-located block mentioned above are each a prediction unit (PU). The prediction unit is a block of an image, and is defined as a data unit for prediction. In HEVC, a coding unit (CU) is defined as a data unit for coding, separately from the prediction unit. The prediction unit is a block in the coding unit. In the following description, the term "block" may be replaced with "prediction unit" or "coding unit". [0018] The coding unit and the prediction unit are not fixed !in size. For example, one picture may include a plurality of coding units of various sizes, and one picture may include a plurality of prediction units of various sizes. [0019] This can cause a situation where a block that exactly matches an area of the current block is not defined in the co-located picture. Accordingly, in HEVC, the co-located block is selected from a plurality of blocks included in the co-located picture by a predetermined selection method. [0020] The temporal motion vector predictor is generated byjscaling the motion vector of the selected co-located block based on a POC (Picture Order Count) distance. POCs are ordinal numbers assigned to pictures in display order. A POC distance corresponds to a temporal distance between two pictures. Scaling based on; a POC v distance is also referred to as POC-based scaling. Expression 1 below is an arithmetic expression for performing POC-based scaling; on the motion vector of the co-located block. [0021] pmv = (tb/td) x colmv •■■ (Expression 1). [0022] Here, colmv is the motion vector of the co-located block, pmv ; is the temporal motion vector predictor derived from the motion vector of the co-located block, tb is a signed POC distance, representing a difference between the current picture and the current reference picture, td is a signed POC distance, representing a difference ; between the co-located picture and the co-located reference picture. [0023] In the case where a valid temporal motion vector predictor is present, the temporal motion vector predictor is inserted into an ordered list of current motion vector candidates. The motion; vector used for coding the current block is selected from the ordered list of current motion vector candidates. The selected motion vector is indicated by a parameter in a bitstream. [0024] FIG. 1 is a flowchart showing an operation of an image; coding apparatus according to a reference example. In particular,- FIG. 1 shows a process of coding an image by inter prediction. [0025] First, the image coding apparatus classifies each of reference pictures as a short-term reference picture or a long-term reference picture (Step S101). The image coding apparatus writes information indicating the classification of each of the reference pictures, to a header of the bitstream (Step S102). [0026] Next, the image coding apparatus identifies the jcurrent 1 reference picture (Step S103). The image coding apparatus then derives the current motion vector (Step S104). A derivation process will be described in detail later. [0027] Following this, the image coding apparatus generates a prediction biock, by performing motion compensation usijng the ; current reference picture and the current motion vector (StepiS105). ; [0028] The image coding apparatus subtracts the prediction block from the current block, to generate a residual block (Step S106). ; Lastly, the image coding apparatus codes the residual block, to generate the bitstream including the coded residual block (Step S107). [0029] FIG. 2 is a flowchart showing an operation of an image decoding apparatus according to the reference example. In particular; FIG. 2 shows a process of decoding an image by inter prediction. [0030] First, the image decoding apparatus obtains the bitstream, and obtains the information indicating the classification of each| of the reference pictures by parsing the header of the bitstream (Step;S201). The image decoding apparatus also obtains the residual block, by parsing the bitstream (Step S202). [0031] Next, the image decoding apparatus identifies the current reference picture (Step S203). The image decoding apparatus then derives the current motion vector (Step S204). A derivation process will be described in detail later. Following this, the image decoding apparatus generates the prediction block, by performing ;motion compensation using the current reference picture and the current motion vector (Step S205). Lastly, the image decoding apparatus adds the prediction block to the residual block, to generate a reconstructed block (Step. $206). [0032] FIG. 3 is a flowchart showing details of the derivation process shown in FIGS. 1 and 2. The following describes the operation of the image coding apparatus. The operation of the image decoding apparatus is the same as the operation of the image coding apparatus, y';' with "coding" being replaced with "decoding". [0033] First, the image coding apparatus selects the co-locatedipicture (Step S301). Next, the image coding apparatus selects the co-located block in the co-located picture (Step S302). The image coding apparatus then identifies the co-located reference picture and the co-located motion vector (Step S303). After this, the image coding apparatus derives the current motion vector by a derivation scheme that involves POC-based scaling (Step S304). [0034] FIG. 4 is a diagram for explaining the co-located block ;used in the derivation process shown in FIG. 3. The co-located block is selected from a plurality of blocks in the co-located picture. [0035] The co-located picture is different from the current picture that includes the current block. For example, the co-located picture is a picture immediately preceding or immediately following the-current picture in display order. In more detail, for example, the collocated picture is a reference picture listed first in any of two reference^picture lists used for coding of B pictures (bi-predictive coding). [0036] A first block including a sample cO in the co-located picture is a leading candidate for the co-located block, and is also referred to as a primary co-located block. A second block including a sample cl in the co-located picture is a second leading candidate for the co-located block, and is also referred to as a secondary co-located block* [0037] Let (x, y) be coordinates of a top left sample tl in thelcurrent block, w be a width of the current block, and h be a height of the current block. Coordinates of the sample cO are (x + w, y + h). Coordinates of the sample cl are (x + (w/2) - 1, y + (h/2) -;1). [0038] In the case where the first block is not available, thejsecond block is selected as the co-located block. Examples of the case where the first block is not available include the case where the first block is not present because the current block is located rightmost or : bottommost in the picture, and the case where the first block is coded : by intra prediction. [0039] The following describes a more specific example of the process of deriving the temporal motion vector predictor as the current motion vector with reference to FIG. 3 again. [0040] First, the image coding apparatus selects the co-located-picture (Step S301). Next, the image coding apparatus selects the co-located block (Step S302). In the case where the first block including the sample cO shown in FIG. 4 is available, the first block is selected as the co-located block. In the case where the first block is not available and the second block including the sample cl shown in I FIG. 4 is available, the second block is selected as the collocated ■ block. [0041] In the case where the available co-located block is selected, the image coding apparatus sets the temporal motion vector predictor as available. In the case where the available co-located block is not selected, the image coding apparatus sets the temporal motion vector predictor as not available. [0042] In the case where the temporal motion vector predictor is set as available, the image coding apparatus identifies the co-located; motion vector as a base motion vector. The image coding apparatus also identifies the co-located, reference picture (Step S303). Thei image coding apparatus then derives the temporal motion vector predictor from the base motion vector by scaling according to Expression 1 (Step S304). [0043] Through the process described above, the image! Coding apparatus and the image decoding apparatus each derive the temporal motion vector predictor as the current motion vector. [0044] There are, however, cases where it is difficult to derive the appropriate current motion vector, depending on the relations I between the current picture, the current reference picture, the ; co-located picture, and the co-located reference picture. [0045] For instance, in the case where the current reference picture is a long-term reference picture, there is a possibility that the temporal distance between the current reference picture and the current picture is long. In the case where the co-located reference pictiire is a ; long-term reference picture, there is a possibility that the temporal distance between the co-located reference picture and the co-jlocated picture is long. [0046] These cases incur a possibility that an extremely large or small current motion vector is generated as a result of POC-based scaling. This causes degradation in prediction accuracy and degradation in coding efficiency. In particular, the extremely large or small Current motion vector cannot be appropriately expressed with a fixed humber of bits, leading to significant prediction accuracy degradation and coding efficiency degradation. [0047] An image coding method according to an aspect of the present invention is an image coding method of coding each of blocks of pictures, the image coding method including: deriving a candidate for a motion vector of a current block to be coded, from a motion vector of a co-located block which is a block included in a picture different from a picture that includes the current block; adding the derived candidate to a list; selecting the motion vector of the current block fromithe list to which the candidate is added; and coding the current block using W the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the co-located block is a long-term reference picture or a short-term reference picture; deriving the candidate from the motion vector of the co-located block by a first derivation scheme that does notiinvolve scaling based on a temporal distance, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate from the motion vector of the co-located block by a second derivation scheme that involves scaling based on a temporal distance, in the case of determining that each \ of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0048] Thus, the candidate for the current motion vector is; appropriately derived without being extremely large or small. This contributes to improved prediction accuracy and improved! coding efficiency. [0049] For example, in the deriving: the deriving of the candidate from the motion vector of the co-located block may not be performed in the case of determining that one of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture and the other one of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture; and the deriving of the candidate from the motion vector of the co-located block may be performed in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture or in the case of determining that each; of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0050] Thus, in the case where low prediction accuracy is expected, the candidate for the current motion vector is not derived from the! motion vector of the co-located block. Prediction accuracy degradation can be prevented in this way. [0051] For example, the coding may further include coding information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and information indicating whether the reference picture of the collocated block is a long-term reference picture or a short-term reference picture. [0052] Thus, the information indicating, for each reference picture, whether the reference picture is a long-term reference picture or a short-term reference picture is provided from the coding side to the decoding side. This enables the coding side and the decoding; side to obtain the same determination result and perform the same process. [0053] For example, the deriving may include: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, using a temporal distance between the reference picture of the current block and the picture that includes the current block; and determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, using a temporal distance between the reference picture of the co-located block and the picture that includes the co-located block. [0054] Thus, for each reference picture, whether the reference^ picture is a long-term reference picture or a short-term reference picture is simply and appropriately determined based on the temporal distance. [0055] For example, the deriving may include determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the co-located block is coded. [0056] Thus, whether the reference picture of the co-located blbck is a long-term reference picture or a short-term reference pidture is determined more accurately. [0057] For example, the deriving may include determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the current block is coded. [0058] Thus, information indicating whether the reference picture of the co-located block is a long-term reference picture or a sholrt-term reference picture need not be retained for a long time. [0059] For example, the deriving may include: deriving theimotion vector of the co-located block as the candidate, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate by scaling the motion vectolr of the co-located block using a ratio, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture, the ratiio being a ratio of a temporal distance between the reference picture of the current block and the picture that includes the current block to a temporal distance between the reference picture of the collocated block and the picture that includes the co-located block. [0060] Thus, in the case where the two reference pictures are! each a long-term reference picture, scaling is omitted, with it being possible to reduce computation. In the case where the two reference pictures are each a short-term reference picture, the candidate for theicurrent motion vector is appropriately derived based on the temporal distance. [0061] For example, the deriving may further include, without deriving the candidate from the co-located block, selecting another co-located block and deriving the candidate from a motion vector of thie other co-located block by the second derivation scheme, in the case of determining that the reference picture of the current blotk is a short-term reference picture and the reference picture !of the co-located block is a long-term reference picture, the other co-ilocated block being coded with reference to a short-term reference picture. [0062] Thus, the block for deriving the candidate of high prediction accuracy is selected. This contributes to improved prediction accuracy. [0063] Moreover, an image decoding method according to an aspect of the present invention is an image decoding method of decodingjeach of blocks of pictures, the image decoding method including: deriving a candidate for a motion vector of a current block to be decoded; from a motion vector of a co-located block which is a block included in a picture different from a picture that includes the current block;; adding the derived candidate to a list; selecting the motion vector of the current block from the list to which the candidate is added; and decoding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the co-located block is a long-term : reference picture or a short-term reference picture; deriving the candidate from the motion vector of the co-located block by a first derivation scheme that does not involve scaling based on a temporal distance, in the case of determining that each of the reference; picture of the current block and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate from the motion vector of the co-located block by a second derivation scheme that involves scaling based on a temporal distance, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0064] Thus, the candidate for the current motion vettor is ; appropriately derived without being extremely large or smalj. This contributes to improved prediction accuracy and improved; coding efficiency. [0065] For example, in the deriving: the deriving of the candidate from the motion vector of the co-located block may not be performed in the ■ case of determining that one of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture and the other one of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture; and the deriving of the candidate from the motion vector of the co-located block may be performed in the case of determining that each of the reference picture of the ^current block and the reference picture of the co-located block is a lorig-term reference picture or in the case of determining that each; of the reference picture of the current block and the reference picture of the t co-located block is a short-term reference picture. [0066] Thus, in the case where low prediction accuracy is expected, the candidate for the current motion vector is not derived from the motion vector of the co-located block. Prediction accuracy degradation can be prevented in this way. [0067] For example, the decoding may further include decoding information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and information indicating whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, using the information indicating whether the reference picture of the current bloick is a :: ;/ ■ ■. long-term reference picture or a short-term reference picture; and determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, using the information indicating whether the reference picture; of the co-located block is a long-term reference picture or a shoirt-term ; reference picture. [0068] Thus, the information indicating, for each reference picture, whether the reference picture is a long-term reference picture or a short-term reference picture is provided from the coding side) to the decoding side. This enables the coding side and the decoding! side to obtain the same determination result and perform the same process. [0069] For example, the deriving may include: determining whether the reference picture of the current block is a long-term reference M picture or a short-term reference picture, using a temporal distance ■ : between the reference picture of the current block and the picture that includes the current block; and determining whether the reference picture of the co-located block is a long-term reference picture or a * ;■■''' short-term reference picture, using a temporal distance between the reference picture of the co-located block and the picture that includes the co-located block. [0070] Thus, for each reference picture, whether the reference; picture is a long-term reference picture or a short-term reference picture is simply and appropriately determined based on the temporal distance. [0071] For example, the deriving may include determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the co-located block is decoded. [0072] Thus, whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture is determined more accurately. [0073] For example, the deriving may include determining whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the current block is decoded. [0074] Thus, information indicating whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture need not be retained for a long time. [0075] For example, the deriving may include: deriving the;motion vector of the co-located block as the candidate, in the case of determining that each of the reference picture of the current blbck and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate by scaling the motion vector of the co-located block using a ratio, in the case of determining thatjeach of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture, the ratiio being a ratio of a temporal distance between the reference picture of the current block and the picture that includes the current block to a temporal distance between the reference picture of the co-located block and the picture that includes the co-located block. [0076] Thus, in the case where the two reference pictures are each a long-term reference picture, scaling is omitted, with it being possible : to reduce computation. In the case where the two reference pictures are each a short-term reference picture, the candidate for the'current motion vector is appropriately derived based on the temporal distance. [0077] For example, the deriving may further include, without deriving the candidate from the co-located block, selecting another co-jlocated block and deriving the candidate from a motion vector of the other co-located block by the second derivation scheme, in the base of determining that the reference picture of the current block is a : short-term reference picture and the reference picture ;of the co-located block is a long-term reference picture, the other collocated block being decoded with reference to a short-term reference picture. [0078] Thus, the block for deriving the candidate of high prediction accuracy is selected. This contributes to improved prediction accuracy. [0079] Moreover, a content providing method according to an aspect of the present invention is a content providing method of transmitting, from a server in which image data coded by the image coding method described above is recorded, the image data in response to a request from an external terminal. [0080] These general and specific aspects may be implemented;using a system, an apparatus, an integrated circuit, a computer program, or a non-transitory computer-readable recording medium such as a CD-ROM, or any combination of systems, apparatuses, methods, integrated circuits, computer programs, and recording media.: [0081] Hereinafter, certain exemplary embodiments are described in greater detail with reference to the accompanying Drawings. Each of the exemplary embodiments described below shows a general or specific example. The numerical values, shapes, materials, structural elements, the arrangement and connection of the structural elements, steps, the processing order of the steps etc. shown in the following exemplary embodiments are mere examples, and therefore do not limit the scope of the present invention. Therefore, among the structural elements in the following exemplary embodiments, structural elements not recited in any one of the independent claims are described as arbitrary structural elements. [0082] [Embodiment 1] FIG. 5 is a block diagram of an image coding apparatus according to Embodiment 1. An image coding apparatus 500J shown in FIG. 5 codes an image on a block basis, and outputs a bitstream including the coded image. In detail, the image coding apparatus 500 includes a subtracting unit 501, a transforming unit 502, a quantizing unit 503, an entropy coder 504, an inverse quantizing unit $05, an inverse transforming unit 506, an adding unit 507, a block memory 508, a picture memory 509, an intra predicting unit 510, an inter predicting unit 511, and a selecting unit 512. [0083] The subtracting unit 501 subtracts a prediction image from an image provided to the image coding apparatus 500, to generate a differential image. The transforming unit 502 frequency-transforms the differential image generated by the subtracting unit 501, to generate a plurality of frequency coefficients. The quantizing unit 503 quantizes the plurality of frequency coefficients generated by the transforming unit 502, to generate a plurality of quantization; coefficients. The entropy coder 504 codes the plurality of quantization coefficients generated by the quantizing unit 503, to generate a bitstream. * [0084] The inverse quantizing unit 505 inverse-quantizes the plurality of quantization coefficients generated by the quantizing unit ;503, to restore the plurality of frequency coefficients. The Jnverse transforming unit 506 inverse-frequency-transforms the plurality of; frequency coefficients restored by the inverse quantizing unit 1505, to restore the differential image. The adding unit 507 adds the prediction image to the differential image restored by the inverse transforming unit 506, to restore (reconstruct) the image. The adding unit 507 stores the restored image (reconstructed imiage) in the block memory 508 and the picture memory 509. [0085] The block memory 508 is a memory for storing the image restored by the adding unit 507, on a block basis. The picture memory 509 is a memory for storing the image restored by the; adding unit 507, on a picture basis. [0086] The intra predicting unit 510 performs intra prediction by referencing to the block memory 508. That is, the intra predicting unit 510 predicts a pixel value in a picture from another pixel value in the picture. The intra predicting unit 510 thus generates the prediction image. The inter predicting unit 511 performjs inter r prediction by referencing to the picture memory 509. That: is, the '■.-]■■■ inter predicting unit 511 predicts a pixel value in a picture from a pixel value in another picture. The inter predicting unit 511 thus generates the prediction image. [0087] The selecting unit 512 selects any of the prediction; image generated by the intra predicting unit 510 and the prediction image generated by the inter predicting unit 511, and outputs the selected prediction image to the subtracting unit 501 and the adding unit 507. [0088] Though not shown in FIG. 5, the image coding apparatus 500 may include a deblocking filtering unit. The deblocking filtering unit may perform a deblocking filtering process on the image restored by m the adding unit 507, to remove noise near block boundaries. The image coding apparatus 500 may also include a controlling unit that controls each process in the image coding apparatus 500. [0089] FIG. 6 is a block diagram of an image decoding apparatus according to this embodiment. An image decoding apparatus 600 shown in FIG. 6 obtains the bitstream, and decodes the image on a block basis. In detail, the image decoding apparatus 600 includes an entropy decoder 601, an inverse quantizing unit 602, an inverse transforming unit 603, an adding unit 604, a block memory; 605, a picture memory 606, an intra predicting unit 607, an inter predicting unit 608, and a selecting unit 609. [0090] The entropy decoder 601 decodes the coded plurality of quantization coefficients included in the bitstream. The iinverse quantizing unit 602 inverse-quantizes the plurality of quantization :, coefficients decoded by the entropy decoder 601, to restore the plurality of frequency coefficients. The inverse transforming unit 603 ; inverse-frequency-transforms the plurality of frequency coefficients ; restored by the inverse quantizing unit 602, to restore the differential image. [0091] The adding unit 604 adds the prediction image to the differential image restored by the inverse transforming unit 603, to irestore (reconstruct) the image. The adding unit 604 outputs the restored : image (reconstructed image). The adding unit 604 also stares the restored image in the block memory 605 and the picture memory 606. [0092] The block memory 605 is a memory for storing the; image restored by the adding unit 604, on a block basis. The ipicture } memory 606 is a memory for storing the image restored by the adding i unit 604, on a picture basis. [0093] The intra predicting unit 607 performs intra prediction by referencing to the block memory 605. That is, the intra predicting unit 607 predicts a pixel value in a picture from another pixel value in the picture. The intra predicting unit 607 thus generates the prediction image. The inter predicting unit 608 performs inter prediction by referencing to the picture memory 606. That; is, the inter predicting unit 608 predicts a pixel value in a picture from a pixel value in another picture. The inter predicting unit 608 thus generates the prediction image. [0094] The selecting unit 609 selects any of the prediction; image generated by the intra predicting unit 607 and the prediction image generated by the inter predicting unit 608, and outputs the selected prediction image to the adding unit 604. [0095] Though not shown in FIG. 6, the image decoding apparatus 600 may include a deblocking filtering unit. The deblocking filtering unit may perform a deblocking filtering process on the image restbred by the adding unit 604, to remove noise near block boundaries. The image decoding apparatus 600 may also include a controlling unit that controls each process in the image decoding apparatus 600. [0096] The coding process and the decoding process mentioned above are performed on a coding unit basis. The transformation process, the quantization process, the inverse transformation process, and the inverse quantization process are performed on a transform uijiit (TU) basis where the transform unit is included in the coding un|t. The i; prediction process is performed on a prediction unit basis where the prediction unit is included in the coding unit. [0097] FIG. 7 is a flowchart showing an operation of the image; coding •■'; apparatus 500 shown in FIG. 5. In particular, FIG. 7 shows a process of coding an image by inter prediction. [0098] First, the inter predicting unit 511 classifies each of reference pictures as a short-term reference picture or a long-term reference picture (Step S701). [0099] The long-term reference picture is a reference picture suitable for long-term use. The long-term reference picture is defined as a reference picture for longer use than the short-term reference picture. Accordingly, there is a high possibility that the long-term reference picture is retained in the picture memory 509 for a long time. The long-term reference picture is designated by an absolute PQC that does not depend on the current picture. Meanwhile, the shojrt-term reference picture is designated by a POC relative to the current picture. [0100] Next, the entropy coder 504 writes information indicating the classification of each of the reference pictures, to a header; of the bitstream (Step S702). That is, the entropy coder 504! writes v V information indicating, for each of the reference pictures, whether the reference picture is a long-term reference picture or a short-term reference picture. [0101] Following this, the inter predicting unit 511 identifies the ;: reference picture of the current block to be coded (to be predicted) (Step S703). The inter predicting unit 511 may identify a reference picture of a block adjacent to the current block, as the current reference picture. Alternatively, the inter predicting unit 511 may identify the current reference picture by a predetermined reference index. The inter predicting unit 511 then derives the current;motion }\-vector (Step S704). A derivation process will be described ih detail later. [0102] The inter predicting unit 511 generates the prediction btack, by performing motion compensation using the current reference;picture ; and the current motion vector (Step S705). After th;is, the : subtracting unit 501 subtracts the prediction block from the-current ! block (original image), to generate the residual block (Step|S706). Lastly, the entropy coder 504 codes the residual block, to generate the bitstream including the residual block (Step S707). [0103] FIG. 8 is a flowchart showing an operation of the: image decoding apparatus 600 shown in FIG. 6. In particular, FIG. $ shows a process of decoding an image by inter prediction. [0104] First, the entropy decoder 601 obtains the bitstream, and obtains the information indicating the classification of each of the reference pictures by parsing the header of the bitstream (Step S801). That is, the entropy decoder 601 obtains the information indicating, for each of the reference pictures, whether the reference picture is a long-term reference picture or a short-term reference picture. The entropy decoder 601 also obtains the residual block, by parsing the bitstream (Step S802). [0105] Next, the inter predicting unit 608 identifies the jcurrent : : reference picture (Step S803). The inter predicting unit 608 may identify a reference picture of a block adjacent to the current block, as the current reference picture. Alternatively, the inter predicting unit 608 may identify the current reference picture by a predetermined reference index. [0106] Following this, the inter predicting unit 608 derives the jcurrent motion vector (Step S804). A derivation process will be described in detail later. The inter predicting unit 608 then generates the prediction block, by performing motion compensation usiing the current reference picture and the current motion vector (Step; S805). Lastly, the adding unit 604 adds the prediction block to the itesidual block, to generate the reconstructed block (Step S806). [0107] FIG. 9 is a flowchart showing details of the derivation process shown in FIGS. 7 and 8. The following mainly describes the operation of the inter predicting unit 511 shown in FIG. 5. The operation of the inter predicting unit 608 shown in FIG. 6 is the same as the operation of the inter predicting unit 511 shown in FIG. 5, with "coding" being replaced with "decoding". [0108] First, the inter predicting unit 511 selects the co-located;picture from a plurality of available reference pictures (Step S901). The plurality of available reference pictures are coded pictures, and are retained in the picture memory 509. [0109] Next, the inter predicting unit 511 selects the co-located block in the co-located picture (Step S902). The inter predicting unit 511 then identifies the co-located reference picture and the co-located motion vector (Step S903). [0110] Following this, the inter predicting unit 511 determines whether or not any of the current reference picture and the co-located reference picture is a long-term reference picture (Step S904). In the case of determining that any of the current reference picture and the co-located reference picture is a long-term reference picture (Step S904: Yes), the inter predicting unit 511 derives the currentlmotion vector by a first derivation scheme (Step S905). [0111] The first derivation scheme is a scheme using the co-Hocated motion vector. In more detail, the first derivation scheme is a scheme of directly deriving the co-located motion vector as the current y motion vector, without POC-based scaling. The first derivation scheme may be a scheme of deriving the current motion vdctor by scaling the co-located motion vector at a predetermined ratio; [0112] In the case of determining that none of the current reference picture and the co-located reference picture is a long-term reference picture (Step S904: No), the inter predicting unit 511 derives the ! current motion vector by a second derivation scheme (Step;S906). That is, in the case of determining that the current reference!picture and the co-located reference picture are each a short-term reference picture, the inter predicting unit 511 derives the current motion vector by the second derivation scheme. [0113] * The second derivation scheme is a scheme using the current reference picture, the co-located reference picture, and the collocated motion vector. In more detail, the second derivation scheme is a scheme of deriving the current motion vector by performing POC-based scaling (Expression 1) on the co-located motion vector. [0114] The following describes a more specific example of the process of deriving the current motion vector with reference to FIG. 9 again. The derivation process described earlier may be changed as follows. [0115] First, the inter predicting unit 511 selects the co-located picture (Step S901). In more detail, in the case where a slice ;header parameter slice_type is B and a slice header parameter' collocated_from_IO_flag is 0, a picture RefPicListl[0] is selected as the co-located picture. The picture RefPicListl[0] is a reference picture listed first in an ordered reference picture list Ref Pic List 1. [0116] In the case where the slice header parameter slice_type jis not B or in the case where the slice header parameter i collocated_from_IO_flag is not 0, a picture RefPicList0[0] is Selected as the co-located picture. The picture RefPicListOfO] is a reference picture listed first in an ordered reference picture list RefPicListO. [0117] Next, the inter predicting unit 511 selects the co-locatqd block (Step S902). In the case where the first block including theisample ;; cO shown in FIG. 4 is available, the first block is selected: as the • Jy co-located block. In the case where the first block is not available and the second block including the sample cl shown in Fib. 4 is available, the second block is selected as the co-located block. [0118] In the case where the available co-located block is selected, the inter predicting unit 511 sets the temporal motion vector predictor as available. In the case where the available co-located block is not selected, the inter predicting unit 511 sets the temporal motion vector predictor as not available. [0119] In the case where the temporal motion vector predictor is set as available, the inter predicting unit 511 identifies the co-located:motion vector as the base motion vector. The inter predicting unit 511 also identifies the co-located reference picture (Step S903). In the case where the co-located block has a plurality of motion vectors, that is, in the case where the co-located block is coded using a plurality of motion vectors, the inter predicting unit 511 selects the baselmotion vector according to predetermined priority order. [0120] For example, in the case where the current reference picture is a short-term reference picture, the inter predicting unit 511 may preferentially select a motion vector that points to a location in a short-term reference picture from among the plurality of:motion vectors, as the base motion vector. [0121] In detail, in the case where a motion vector that points to a location in a short-term reference picture is present, thte inter : predicting unit 511 selects the motion vector as the baseimotion vector. In the case where a motion vector that points to a location in a short-term reference picture is not present, the inter predicting unit : 511 selects a motion vector that points to a location in a long-term ; reference picture, as the base motion vector. [0122] After this, in the case where any of the current reference;picture and the co-located reference picture is a long-term reference:picture ; (Step S904: Yes), the inter predicting unit 511 derives trie base motion vector as the temporal motion vector predictor (Step S905). [0123] In the case where none of the two reference pictures is a long-term reference picture (Step S904: No), on the other hajnd, the inter predicting unit 511 derives the temporal motion vector predictor i from the base motion vector by POC-based scaling (Step S906). [0124] As described above, the temporal motion vector predictpr is set * as available or not available. The inter predicting unit 511 inserts the temporal motion vector predictor set as available, into an ordered list of current motion vector candidates. The ordered list holds not only the temporal motion vector predictor but various motion vectors as candidates. [0125] The inter predicting unit 511 selects one motion vector from the ordered list, as the current motion vector. Here, the inter predicting unit 511 selects a motion vector of highest prediction accuracy, for the current block or a motion vector that allows the current block to be coded with highest coding efficiency, from the ordered list. An index corresponding to the selected motion vector is written ;to the bitstream. [0126] Through the process described above, the current motion vector is appropriately derived from the co-located motion vector, without being extremely large or small. This contributes to improved v prediction accuracy and improved coding efficiency. [0127] Note that the status of each reference picture as to whether the reference picture is a long-term reference picture or a short-term reference picture may be changed according to time. For exahnple, a short-term reference picture may later be changed to a loiig-term reference picture, and a long-term reference picture may later be changed to a short-term reference picture. [0128] Moreover, the inter predicting unit 511 may determine whether the co-located reference picture is a long-term reference picture or a : short-term reference picture, in a period during which the collocated •: block is coded. The image coding apparatus 500 may then include an additional memory for holding the determination result from when the co-located block is coded to when the current block is coded.; [0129] In this way, whether the co-located reference picture is a long-term reference picture or a short-term reference picture is ; determined more accurately. [0130] As an alternative, the inter predicting unit 511 may determine whether the co-located reference picture is a long-term reference picture or a short-term reference picture, in a period during which the current block is coded. [0131] In this way, the information of whether the co-located reference picture is a long-term reference picture or a short-term reference picture need not be retained for a long time. [0132] Moreover, the inter predicting unit 511 may determine whether the current reference picture is a long-term reference picture or a ; short-term reference picture, using a temporal distance between the current reference picture and the current picture. [0133] As an example, in the case where the temporal distance between the current reference picture and the current picture is more than a predetermined threshold, the inter predicting unit 511 determines that the current reference picture is a long-term reference picture. In the case where the temporal distance is not more than the predetermined threshold, the inter predicting unit 511 determines that the current reference picture is a short-term reference picture. [0134] Likewise, the inter predicting unit 511 may determine whether ; the co-located reference picture is a long-term reference picture or a short-term reference picture, using a temporal distance between the co-located reference picture and the co-located picture. [0135] As an example, in the case where the temporal distance; W between the co-located reference picture and the co-located pijcture is more than a predetermined threshold, the inter predicting unit 511 determines that the co-located reference picture is a long-term ■ reference picture. In the case where the temporal distance is not more than the predetermined threshold, the inter predicting unit 511 : determines that the co-located reference picture is a short-term reference picture. [0136] The inter predicting unit 608 in the image decoding apparatus 600 may determine, for each reference picture, whether or not the reference picture is a long-term reference picture or a short-term reference picture based on a temporal distance, in the same manner as the inter predicting unit 511 in the image coding apparatus 500. In such a case, the information indicating, for each reference picture, whether the reference picture is a long-term reference picture or a short-term reference picture need not be coded. [0137] Regarding each of the other processes described jin this; embodiment, too, each structural element in the image decoding apparatus 600 performs the same process as the corresponding structural element in the image coding apparatus 500, as a result of which the image coded with high coding efficiency is appropriately decoded. [0138] The operations described above are also applicable to trie other embodiments. Any of the structures and operations describee! in this embodiment may be incorporated in the other embodiments, and any of the structures and operations described in the other embodiments may be incorporated in this embodiment. [0139] [Embodiment 2] An image coding apparatus and an image decoding apparatus according to Embodiment 2 have the same structures as those in Embodiment 1. Hence, the operations of the image coding apparatus; and the image decoding apparatus according to this embodiment are described below, using the structure of the image coding apparatus ; 500 shown in FIG. 5 and the structure of the image decoding apparatus 600 shown in FIG. 6. [0140] The image coding apparatus 500 according to this embodiment performs the operation shown in FIG. 7, as in Embodiment 1. The image decoding apparatus 600 according to this embodiment performs the operation shown in FIG. 8, as in Embodiment 1. This embodiment differs from Embodiment 1 in the current motion vector derivation process. This is described in detail below. [0141] FIG. 10 is a flowchart showing details of the derivation process according to this embodiment. The inter predicting unit 511 according to this embodiment performs the operation shown in iFIG. 10, instead of the operation shown in FIG. 9. The following; mainly describes the operation of the inter predicting unit 511 shown in FIG. 5. The operation of the inter predicting unit 608 shown in F^G. 6 is the same as the operation of the inter predicting unit 511 shown in FIG. 5, with "coding" being replaced with "decoding". [0142] First, the inter predicting unit 511 selects the co-located;picture from the plurality of available reference pictures (Step S1001).; Next, the inter predicting unit 511 selects the co-located block; in the co-located picture (Step S1Q02). The inter predicting unit 511 then identifies the co-located reference picture and the co-located;motion ; vector (Step S1003). [0143] Following this, the inter predicting unit 511 determines whether or not the current reference picture is a long-term reference;picture (Step S1004). In the case of determining that the current reference picture is a long-term reference picture (Step S1004: Yes), the inter predicting unit 511 derives the current motion vector by the first derivation scheme same as in Embodiment 1 (Step S1005). [0144] In the case of determining that the current reference picture is ; not a long-term reference picture (Step S1004: No), thie inters predicting unit 511 determines whether or not the co-jlocated reference picture is a long-term reference picture (Step S1006). [0145] In the case of determining that the co-located reference-picture is not a long-term reference picture (Step S1006: No), the inter predicting unit 511 derives the current motion vector by theisecond derivation scheme same as in Embodiment 1 (Step S1007). That is, in the case of determining that the current reference picture and the co-located reference picture are each a short-term reference picture, the inter predicting unit 511 derives the current motion vector by the second derivation scheme. [0146] In the case of determining that the co-located reference;picture is a long-term reference picture (Step S1006: Yes), the inter predicting unit 511 selects another co-located block in the co-located picture (Step S1008). In the example shown in FIG. 10, a block coded with reference to a short-term reference picture is selected as r the other co-located block. [0147] After this, the inter predicting unit 511 identifies the collocated reference picture and the co-located motion vector corresponding to the other co-located block (Step S1009). The inter predicting unit 511 then derives the current motion vector by the second derivation scheme that uses POC-based scaling (Step S1010). [0148] In detail, in the case where the reference picture of thejcurrent block is a short-term reference picture and the reference picture of the co-located block is a long-term reference picture, the inter predicting unit 511 does not derive the current motion vector from thejmotion vector of the co-located block. The inter predicting unit 511 -instead selects another co-located block coded with reference to a short-term reference picture, and derives the current motion vector from the motion vector of the selected other co-located block. [0149] As an example, in the case where the reference picture of the current block is a short-term reference picture and the reference ;! picture of the co-located block is a long-term reference picture, the inter predicting unit 511 searches for a block coded with reference to : a short-term reference picture. The inter predicting unit 51liselects the block coded with reference to the short-term reference picture, as the other co-located block. [0150] As another example, in the case where the reference picture of the current block is a short-term reference picture and the reference picture of the co-located block is a long-term reference picture, the inter predicting unit 511 first searches for a block coded with reference to a short-term reference picture. [0151] In the case where the block coded with reference:to the short-term reference picture is present, the inter predicting unit 511 selects the block as the other co-located block. In the case where the block coded with reference to the short-term reference picture is not present, the inter predicting unit 511 searches for a block coded with reference to a long-term reference picture. The inter predicting unit ; 511 selects the block coded with reference to the long-term reference ; picture, as the other co-located block. [0152] For example, the inter predicting unit 511 first selects the first block shown in FIG. 4 as the co-located block. In the case where the current reference picture is a short-term reference picture and the co-located reference picture is a long-term reference picture, then the ; inter predicting unit 511 newly selects the second block shown: in FIG. 4 as the co-located block. [0153] In the above-mentioned example, the inter predicting unit 511 may select the second block shown in FIG. 4 as the co-locatejd block only in the case where the reference picture of the second block is a short-term reference picture. The block selected as the co-ilocated block here is not limited to the second block shown in FIG.4:, and a ; block other than the second block may be selected as the collocated block. [0154] FIG. 11 is a diagram for explaining the co-located block according to this embodiment. Samples cO, cl, c2, and c3 in the co-located picture are shown in FIG. 11. The samples cO and cl in FIG. 11 are equal to the samples cO and cl in FIG. 4. Not only the second block including the sample cl but also a third block including the sample c2 or a fourth block including the sample c3 may be selected as the other co-located block. [0155] Coordinates of the sample c2 are (x + w - 1, y + h - 1). Coordinates of the sample c3 are (x + 1, y + 1). [0156] The inter predicting unit 511 determines, for each of the first, second, third, and fourth blocks in this order, whether or not the block ; is available. The inter predicting unit 511 determines the available block as the final co-located block. Examples of the case where the block is not available include the case where the block is not present and the case where the block is coded by intra prediction. [0157] In the case where the current reference picture is a shojrt-term reference picture, the inter predicting unit 511 may determine that a block coded with reference to a long-term reference picture is not ' available. [0158] Though the above describes the example of the co-located block selection method, the co-located block selection method is not limited to the above example. A block including a sample other than the samples cO, cl, c2, and c3 may be selected as the co-located block. Besides, the priority order of the blocks is not limited to the example described in this embodiment. [0159] The following describes a more specific example of the process of deriving the current motion vector with reference to FIG. 10 again. The derivation process described earlier may be changed as follows. [0160] First, the inter predicting unit 511 selects the co-locatedipicture as in Embodiment 1 (Step S1001). Next, the inter predicting unit 511 ¥ selects the first block including the sample cO shown in FIG. 11 as the co-located block, and identifies the co-located reference picture (Steps S1002 and S1003). [0161] Following this, the inter predicting unit 511 determines whether or not the co-located block is available. In the case where the current reference picture is a short-term reference picture and the co-located reference picture is a long-term reference picture, the inter predicting unit 511 determines that the co-located block is not available (Steps S1004 and S1006). [0162] In the case where the co-located block is not available, the inter predicting unit 511 searches for and selects another co-located block which is available (Step S1008). In detail, the inter predicting unit ; 511 selects a block coded with reference to a short-term reference picture, from among the second block including the sample ;cl, the third block including the sample c2, and the fourth block including the sample c3 in FIG. 11. The inter predicting unit 511 then identifies the reference picture of the co-located block (Step S1009). [0163] In the case where the available co-located block is selected, the inter predicting unit 511 sets the temporal motion vector predictor as available. In the case where the available co-located block is not selected, the inter predicting unit 511 sets the temporal motion vector predictor as not available. [0164] In the case where the temporal motion vector predictor is set as available, the inter predicting unit 511 identifies the co-locatedj motion vector as the base motion vector (Steps S1003 and S1009). I In the case where the co-located block has a plurality of motion vectors, that is, in the case where the co-located block is coded using a plurality of motion vectors, the inter predicting unit 511 selects the baselmotion vector according to predetermined priority order as in Embodiment 1. [0165] In the case where any of the current reference picture and the co-located reference picture is a long-term reference picture (Step S1004: Yes), the inter predicting unit 511 derives the base motion vector as the temporal motion vector predictor (Step S1005).: [0166] In the case where none of the current reference picture and the co-located reference picture is a long-term reference picture (Step S1004: No), on the other hand, the inter predicting unit 511 derives the temporal motion vector predictor from the base motion vector by POC-based scaling (Steps S1007 and S1010). [0167] In the case where the temporal motion vector predictor is set as not available, the inter predicting unit 511 does not derive the temporal motion vector predictor. [0168] As in Embodiment 1, the inter predicting unit 511 adds the :;'.; ■'. '■ temporal motion vector predictor set as available, to the list; as the candidate for the current motion vector. The inter predicting unit 511 then selects the current motion vector from the list. [0169] As described above, in this embodiment, in the case where the reference picture of the current block is a short-term referenceipicture and the reference picture of the co-located block is a long-term reference picture, the current motion vector is not derived friom the motion vector of the co-located block. [0170] It is extremely difficult to derive the current motion vector of high prediction accuracy, in the case where one of the ;current reference picture and the co-located reference picture is a lorig-term reference picture and the other one of the current reference;picture and the co-located reference picture is a short-term reference picture. In view of this, the image coding apparatus 500 and the: image decoding apparatus 600 according to this embodiment prevent prediction accuracy degradation by the operation described above. [0171] t [Embodiments] An image coding apparatus and an image decoding apparatus according to Embodiment 3 have the same structures as those in Embodiment 1. Hence, the operations of the image coding apparatus and the image decoding apparatus according to this embodiment are described below, using the structure of the image coding apparatus 500 shown in FIG. 5 and the structure of the image decoding apparatus 600 shown in FIG. 6. [0172] The image coding apparatus 500 according to this embodiment performs the operation shown in FIG. 7, as in Embodiment 1. The image decoding apparatus 600 according to this embodiment performs the operation shown in FIG. 8, as in Embodiment 1. This embodiment differs from Embodiment 1 in the current motion vector derivation process. This is described in detail below. [0173] I FIG. 12 is a flowchart showing details of the derivation process ; according to this embodiment. The inter predicting unit 511 according to this embodiment performs the operation shown in jFIG. 12, instead of the operation shown in FIG. 9. The following; mainly describes the operation of the inter predicting unit 511 shown; in FIG. }..■■ 5. The operation of the inter predicting unit 608 shown in FJG. 6 is the same as the operation of the inter predicting unit 511 shown in FIG.; 5, with "coding" being replaced with "decoding". [0174] First, the inter predicting unit 511 selects the co-located;picture from the plurality of available reference pictures (Step S1201).; Next, i the inter predicting unit 511 selects the co-located block; in the ; co-located picture (Step S1202). The inter predicting unit 5U then identifies the co-located reference picture and the co-located;motion ■: vector (Step S1203). [0175] Following this, the inter predicting unit 511 determines whether ; or not the current reference picture is a long-term reference\picture '■:''[. (Step S1204). In the case of determining that the current reference picture is a long-term reference picture (Step S1204: Yes), the inter predicting unit 511 derives the current motion vector by the first derivation scheme same as in Embodiment 1 (Step S1205). [0176] In the case of determining that the current reference picture is not a long-term reference picture (Step S1204: No), the inter predicting unit 511 determines whether or not the co-located reference picture is a long-term reference picture (Step S1206). [0177] In the case of determining that the co-located reference picture is not a long-term reference picture (Step S1206: No), the inter predicting unit 511 derives the current motion vector by thelsecond derivation scheme same as in Embodiment 1 (Step S1207). That is, : in the case of determining that the current reference picture and the co-located reference picture are each a short-term reference picture, the inter predicting unit 511 derives the current motion vector by the second derivation scheme. [0178] In the case of determining that the co-located reference;picture is a long-term reference picture (Step S1206: Yes), the inter predicting unit 511 selects another co-located picture (Step S1208). The inter predicting unit 511 then selects another co-located block in the other co-located picture (Step S1209). In the example shown in FIG. 12, a block coded with reference to a short-term reference!picture ! is selected as the other co-located block. [0179] After this, the inter predicting unit 511 identifies the collocated ■ reference picture and the co-located motion vector corresponding to the other co-located block (Step S1210). The inter predicting unit 511 then derives the current motion vector by the second derivation scheme that uses POC-based scaling (Step S1211). [0180] In detail, in the case where the reference picture of the Current block is a short-term reference picture and the reference picture of the co-located block is a long-term reference picture, the inter predicting f. unit 511 does not derive the current motion vector from the|motion vector of the co-located block. [0181] The inter predicting unit 511 instead selects another co-ilocated picture. The inter predicting unit 511 further selects another co-located block coded with reference to a short-term reference picture, from the selected other co-located picture. The inter predicting unit 511 derives the current motion vector from the^ motion vector of the selected other co-located block. [0182] As an example, in the case where the current reference;picture is a short-term reference picture and the co-located reference;picture is a long-term reference picture, the inter predicting unit 511 searches ; for a picture that includes a block coded with reference to a short-term reference picture. The inter predicting unit 511 selects thejpicture that includes the block coded with reference to the short-term reference picture, as the other co-located picture. [0183] As another example, in the case where the current reference picture is a short-term reference picture and the co-located reference picture is a long-term reference picture, the inter predicting unit 511 first searches for a picture that includes a block coded with reference to a short-term reference picture. [0184] In the case where the picture that includes the block coded with ; reference to the short-term reference picture is present, the inter predicting unit 511 selects the picture as the other co-located picture. : [0185] In the case where the picture that includes the block coded with reference to the short-term reference picture is not present, the inter predicting unit 511 searches for a picture that includes a block coded with reference to a long-term reference picture. The inter predicting unit 511 selects the picture that includes the block coded with ■ reference to the long-term reference picture, as the other co-ilocated picture. [0186] For example, in the case where the picture RefPicl_istO[0] is the co-located picture, the picture RefPicListl[0] is the other co-;located picture. In the case where the picture RefPicListlfO] is the co-located picture, the picture RefPicl_istO[0] is the other co-located picture. [0187] In other words, the picture listed first in one of the two reference picture lists used for coding of B pictures (bi-predictive coding) is the co-located picture, and the picture listed first in the other one of the two reference picture lists is the other co-located picture. [0188] The following describes a more specific example of the process of deriving the current motion vector with reference to FIG. 12 again, r The derivation process described earlier may be changed as follows. [0189] First, the inter predicting unit 511 selects one of theipicture RefPicListO[0] and the picture RefPicListlfO], as the co-locatedipicture (Step S1201). The inter predicting unit 511 selects, from the selected co-located picture, the first block including the sample cO shown in FIG. 11 as the co-located block, and identifies the co-ilocated reference picture (Steps S1202 and S1203). [0190] Following this, the inter predicting unit 511 determines whether or not the co-located block is available. In the case where the current reference picture is a short-term reference picture and the collocated reference picture is a long-term reference picture, the inter predicting unit 511 determines that the co-located block is not available (Steps S1204 and S1206). [0191] In the case where the co-located block is not available, the inter predicting unit 511 newly selects an available co-located block. For example, the inter predicting unit 511 selects the second block ( including the sample cl shown in FIG. 11, as the co-located block. The inter predicting unit 511 then identifies the co-located reference picture. [0192] In the case where the available co-located block is not selected, the inter predicting unit 511 selects another co-located picture-. Here, the inter predicting unit 511 selects the other one of the-picture RefPicListO[0] and the picture RefPicListl[0], as the co-located:picture (Step S1208). [0193] The inter predicting unit 511 selects, from the selected co-located picture, the first block including the sample cO shown in FIG. 1 as the co-located block, and identifies the co-located reference picture (Steps S1209 and S1210). [0194] Following this, the inter predicting unit 511 determines whether or not the co-located block is available. As in the previous determination, in the case where the current reference picture is a ';':-.' short-term reference picture and the co-located reference picture is a long-term reference picture, the inter predicting unit 511 determines that the co-located block is not available. [0195] In the case where the co-located block is not available, the inter predicting unit 511 newly selects an available co-located block (Step S1209). In detail, the inter predicting unit 511 selects thejsecond block including the sample cl shown in FIG. 11, as the collocated block. The inter predicting unit 511 then identifies the co-jlocated reference picture (Step S1210). [0196] In the case where the available co-located block is eventually selected, the inter predicting unit 511 sets the temporal motion vector predictor as available. In the case where the available co-located block is eventually not selected, the inter predicting unit 511 sets the temporal motion vector predictor as not available. [0197] In the case where the temporal motion vector predictor iis set as available, the inter predicting unit 511 identifies the motion vector of the co-located block as the base motion vector (Steps S1203 and S1210). In the case where the co-located block has a plurality of motion vectors, that is, in the case where the co-located block is coded using a plurality of motion vectors, the inter predicting unit 511 selects the base motion vector according to predetermined priority order as in Embodiment 1. [0198] In the case where any of the current reference picture and the co-located reference picture is a long-term reference picture (Step S1204: Yes), the inter predicting unit 511 derives the base motion vector as the temporal motion vector predictor (Step S1205).; [0199] In the case where none of the current reference picture and the co-located reference picture is a long-term reference picture (Step S1204: No), on the other hand, the inter predicting unit 511 derives the temporal motion vector predictor from the base motion vector by POC-based scaling (Steps S1207 and S1211). [0200] In the case where the temporal motion vector predictor Xs set as not available, the inter predicting unit 511 does not derive the temporal motion vector predictor. [0201] As in Embodiment 1, the inter predicting unit 511 adds the temporal motion vector predictor set as available, to the list as the candidate for the current motion vector. The inter predicting unit 511 then selects the current motion vector from the list. [0202] As described above, the image coding apparatus 500 and the image decoding apparatus 600 according to this embodiment select the block suitable for current motion vector derivation from a plurality of pictures, and derive the current motion vector from theimotion vector of the selected block. This contributes to improved; coding efficiency. [0203] r [Embodiment 4] Embodiment 4 confirmatorily describes the characteristic structures and the characteristic procedures included in Embodiments 1 to 3. [0204] FIG. 13A is a block diagram of an image coding apparatus according to this embodiment. An image coding apparatus 1300 shown in FIG. 13A codes each of blocks of pictures. The imagd coding apparatus 1300 includes a deriving unit 1301, an adding unit 1302, a selecting unit 1303, and a coder 1304. [0205] For example, the deriving unit 1301, the adding unit 1302, and the selecting unit 1303 correspond to the inter predicting unit 511 shown in FIG. 5 and the like, and the coder 1304 corresponds to the entropy coder 504 shown in FIG. 5 and the like. [0206] FIG. 13B is a flowchart showing an operation of the image coding apparatus 1300 shown in FIG. 13A. [0207] The deriving unit 1301 derives a candidate for a motion vector of a current block, from a motion vector of a co-located block (Step ; S1301). The co-located block is a block included in a picture different from a picture that includes the current block to be coded. [0208] In the derivation of the candidate, the deriving unit 1301 determines whether a reference picture of the current block is a ; long-term reference picture or a short-term reference picturfe. The deriving unit 1301 also determines whether a reference picture of the co-located block is a long-term reference picture or a short-term reference picture. [0209] ; In the case of determining that the reference picture of the ; current block and the reference picture of the co-located block are each a long-term reference picture, the deriving unit 1301 derives the candidate from the motion vector of the co-located block by a first r derivation scheme. The first derivation scheme is a derivation scheme that does not involve scaling based on a temporal distance. [0210] In the case of determining that the reference picture! of the current block and the reference picture of the co-located block are each a short-term reference picture, on the other hand, the deriving unit 1301 derives the candidate from the motion vector; of the co-located block by a second derivation scheme. The second derivation scheme is a derivation scheme that involves scaling based on a temporal distance. [0211] The adding unit 1302 adds the derived candidate to a list (Step S1302). The selecting unit 1303 selects the motion vector of the current block from the list to which the candidate is added (Step S1303). [0212] | The coder 1304 codes the current block using the s-elected motion vector and the reference picture of the current block (Step S1304). [0213] FIG. 14A is a block diagram of an image decoding apparatus 0 according to this embodiment. An image decoding apparatus 1400 ; shown in FIG. 14A decodes each of blocks of pictures. The image decoding apparatus 1400 includes a deriving unit 1401, an adding unit 1402, a selecting unit 1403, and a decoder 1404. [0214] For example, the deriving unit 1401, the adding unit 14D2, and the selecting unit 1403 correspond to the inter predicting uinit 608 ■ shown in FIG. 6 and the like, and the decoder 1404 correspondjs to the entropy decoder 601 shown in FIG. 6 and the like. [0215] FIG. 14B is a flowchart showing an operation of thes image decoding apparatus 1400 shown in FIG. 14A. [0216] The deriving unit 1401 derives a candidate for a motion vector of a current block, from a motion vector of a co-located block (Step S1401). The co-located block is a block included in a picture different from a picture that includes a current block to be decoded. [0217] In the derivation of the candidate, the deriving unit 1401 determines whether a reference picture of the current block is a long-term reference picture or a short-term reference picture. The deriving unit 1401 also determines whether a reference picture of the co-located block is a long-term reference picture or a short-term reference picture. [0218] In the case of determining that the reference picture of the current block and the reference picture of the co-located block are each a long-term reference picture, the deriving unit 1401 derives the candidate from the motion vector of the co-located block by a first derivation scheme. The first derivation scheme is a derivation scheme that does not involve scaling based on a temporal distance. [0219] In the case of determining that the reference picture of the current block and the reference picture of the co-located block are each a short-term reference picture, on the other hand, the deriving unit 1401 derives the candidate from the motion vector! of the co-located block by a second derivation scheme. The isecond V derivation scheme is a derivation scheme that involves scaling based on a temporal distance. [0220] The adding unit 1402 adds the derived candidate to a list (Step S1402). The selecting unit 1403 selects the motion vector of the ; current block from the list to which the candidate is added (Step Y \. S1403). [0221] The decoder 1404 decodes the current block using the selected motion vector and the reference picture of the current block (Step S1404). [0222] Through the process described above, the candidate ;for the current motion vector is appropriately derived from the motion vector of the co-located block, without being extremely large or small. This contributes to improved prediction accuracy and improved! coding efficiency. [0223] Here, the deriving units 1301 and 1401 may each not derive the candidate from the motion vector of the co-located block, in the case of determining that one of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture and the other one of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0224] In this case, the deriving units 1301 and 1401 may eachifurther select another co-located block coded or decoded with reference to a short-term reference picture, and derive the candidate from the other co-located block by the second derivation scheme. As an alternative, : the deriving units 1301 and 1401 may each derive the candidate by another derivation scheme. As another alternative, the deriving units 1301 and 1401 may each eventually not derive the candidate corresponding to the temporal motion vector predictor. [0225] The deriving units 1301 and 1401 may determine whether the : reference picture of the current block is a long-term referencelpicture or a short-term reference picture, using a temporal distance between : the reference picture of the current block and the picture that includes ;; the current block. [0226] The deriving units 1301 and 1401 may each determine Whether / the reference picture of the co-located block is a long-term reference r picture or a short-term reference picture, using a temporal distance between the reference picture of the co-located block and theipicture that includes the co-located block. [0227] The deriving units 1301 and 1401 may each determine whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the co-located block is coded or decoded. [0228] The deriving units 1301 and 1401 may each determine whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture, in a period during which the current block is coded or decoded. [0229] The first derivation scheme may be a scheme of deriving the motion vector of the co-located block as the candidate. Thesecond derivation scheme may be a scheme of deriving the candidate by scaling the motion vector of the co-located block using a ratio of the temporal distance between the reference picture of the current block and the picture that includes the current block to the temporal distance between the reference picture of the co-located block jand the picture that includes the co-located block. [0230] The coder 1304 may further code information indicating whether the reference picture of the current block is a long-term ; reference picture or a short-term reference picture, and information ; indicating whether the reference picture of the co-located block is a ; long-term reference picture or a short-term reference picture! [0231] The decoder 1404 may further decode information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and information W indicating whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture; [0232] The deriving unit 1401 may then determine whether the reference picture of the current block is a long-term reference;picture ; or a short-term reference picture using the decoded information, and determine whether the reference picture of the co-located block is a long-term reference picture or a short-term reference picture using the decoded information. [0233] Information indicating classification of each reference;picture may be stored, as a parameter, in a bitstream at a location described below. [0234] FIG. 15A is a diagram showing a first example of the storage location of the parameter indicating the reference ;picture classification. As shown in FIG. 15A, the parameter indicating the reference picture classification may be stored in a sequence header. The sequence header is also referred to as a sequence parameter set. [0235] FIG. 15B is a diagram showing a second example of the storage location of the parameter indicating the reference jpicture ; classification. As shown in FIG. 15B, the parameter indicating the reference picture classification may be stored in a picture header. The picture header is also referred to as a picture parameter set. [0236] FIG. 15C is a diagram showing a third example of the Storage location of the parameter indicating the reference ; picture classification. As shown in FIG. 15C, the parameter indicating the reference picture classification may be stored in a slice headejr. [0237] Information indicating a prediction mode (inter prediction or intra prediction) may be stored, as a parameter, in the bitstream at a location described below. [0238] FIG. 16 is a diagram showing an example of the storage Hocation of the parameter indicating the prediction mode. As shown in HG. 16, the parameter may be stored in a CU header (coding unit Neader). The parameter indicates whether a prediction unit in a coding; unit is coded by inter prediction or intra prediction. This parameter jmay be used to determine whether or not the co-located block is available. [0239] Each of the structural elements in each of the above-described embodiments may be configured in the form of an exclusive hardware product, or may be realized by executing a software program suitable for the structural element. Each of the structural elements may be realized by means of a program executing unit, such as a CPU and a processor, reading and executing the software program record-ed on a recording medium such as a hard disk or a semiconductor memory. Here, the software program for realizing the image coding apparatus and the like according to each of the embodiments is a program described below. [0240] The program causes a computer to execute an image; coding method of coding each of blocks of pictures, the image coding method including: deriving a candidate for a motion vector of a current block to be coded, from a motion vector of a co-located block which is; a block included in a picture different from a picture that includes the icurrent block; adding the derived candidate to a list; selecting theimotion V vector of the current block from the list to which the candidate is added; and coding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the icurrent block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the co-located block is a long-term reference picture or a short-term reference picture; ; deriving the candidate from the motion vector of the co-located block by a first derivation scheme that does not involve scaling based on a temporal distance, in the case of determining that each \ of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture; and denying the candidate from the motion vector of the co-located block by alsecond derivation scheme that involves scaling based on a temporal distance, v | in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0241] The program may cause the computer to execute an image decoding method of decoding each of blocks of pictures, the image decoding method including: deriving a candidate for a motion vector of a current block to be decoded, from a motion vector of a co-located block which is a block included in a picture different from a picture that includes the current block; adding the derived candidate to a list; selecting the motion vector of the current block from the list to which the candidate is added; and decoding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a reference picture of the co-located block is a long-term reference picture or a short-term reference picture; deriving the candidate from the motion vector of the co-located block by a first derivation scheme that does notjinvolve scaling based on a temporal distance, in the case of determining that each of the reference picture of the current block and the reference picture of the co-located block is a long-term reference picture; and deriving the candidate from the motion vector of the co-located block by a second derivation scheme that involves scaling baseb on a temporal distance, in the case of determining that each; of the reference picture of the current block and the reference picture of the co-located block is a short-term reference picture. [0242] Each of the structural elements may be a circuit. These-circuits may wholly constitute one circuit, or be separate circuits. Each of the structural elements may be realized by a general-purpose processor or realized by a special-purpose processor. [0243] The image coding apparatuses according to one or more embodiments have been described above, but the scope of the present invention is not limited to these embodiments. Those skilled in the art will readily appreciate that various modifications may be made in these exemplary embodiments and variations may be obtained by arbitrarily combining structural elements of different embodiments, t without materially departing from the scope of the present invention. Accordingly, all such modifications and variations may be substantially included in the one or more embodiments disclosed herein. [0244] For example, an image coding and decoding apparatus may include the image coding apparatus and the image decoding apparatus. A process executed by a specific processing unit may be executed by another processing unit. Processes may be executed in different order, and two or more processes may be executed in parallel. [0245] [Embodiment 5] The processing described in each of embodiments can be simply implemented in an independent computer systerfi, by recording, in a recording medium, a program for implementing the configurations of the moving picture coding method (image coding method) and the moving picture decoding method (image decoding method) described in each of embodiments. The recording media may be any recording media as long as the program can be recorded, such as a magnetic disk, an optical disk, a magnetic optical dijsk, an IC card, and a semiconductor memory. [0246] Hereinafter, the applications to the moving picture coding method (image coding method) and the moving picture decoding method (image decoding method) described in each of embodiments and systems using thereof will be described. The system lhas a feature of having an image coding and decoding apparatus that includes an image coding apparatus using the image coding method and an image decoding apparatus using the image decoding method. Other configurations in the system can be changed as appropriate depending on the cases. [0247] FIG. 17 illustrates an overall configuration of a content providing system exlOO for implementing content distribution services. The area for providing communication services is divided into cells of desired size, and base stations exl06, exl07, exl08, exl09, and exllO which are fixed wireless stations are placed in each of the cells. [0248] The content providing system exlOO is connected to devices, such as a computer exlll, a personal digital assistant (PDA) exll2, a camera exll3, a cellular phone exll4 and a game machine exll5, via the Internet exlOl, an Internet service provider exi;02, a telephone network exl04, as well as the base stations exi06 to exllO, respectively. [0249] However, the configuration of the content providing system exlOO is not limited to the configuration shown in FIG. 17, iand a combination in which any of the elements are connected is acceptable. In addition, each device may be directly connected to the telephone network exl04, rather than via the base stations exl06 to exllO which are the fixed wireless stations. Furthermore, the devices may be interconnected to each other via a short distance wireless communication and others. [0250] The camera exll3, such as a digital video camera, is capable of capturing video. A camera exll6, such as a digital camera, is capable of capturing both still images and video. Furthermore, the cellular phone exll4 may be the one that meets any of the standards such as Global System for Mobile Communications (GSM) (registered trademark), Code Division Multiple Access (CDMA), Widebandl-Code Division Multiple Access (W-CDMA), Long Term Evolution (LTEJ), and High Speed Packet Access (HSPA). Alternatively, the cellular phone exll4 may be a Personal Handyphone System (PHS). [0251] In the content providing system exlOO, a streaming server exl03 is connected to the camera exll3 and others via the telephone network exl04 and the base station exl09, which enables distribution of images of a live show and others. In such a distribution, a content (for example, video of a music live show) captured by the user using the camera exll3 is coded as described above in each of embodiments (i.e., the camera functions as the image coding apparatus according to an aspect of the present invention), and the coded content is transmitted to the streaming server exl03. On the other hand, the streaming server exl03 carries out stream distribution of the transmitted content data to the clients upon their requests. The clients include the computer exlll, the PDA exll2, the camera exll3, the cellular phone exll4, and the game machine exll5 that are capable of decoding the above-mentioned coded data. Each of the devices that; have received the distributed data decodes and reproduces the coded data (i.e., functions as the image decoding apparatus according ;to an aspect of the present invention). [0252] The captured data may be coded by the camera exll3 or the streaming server exl03 that transmits the data, or the coding processes may be shared between the camera exll3 an£i the streaming server exl03. Similarly, the distributed data mjay be decoded by the clients or the streaming server exl03, or the decoding processes may be shared between the clients and the streaming server exl03. Furthermore, the data of the still images and video captured by not only the camera exll3 but also the camera exll6 may be transmitted to the streaming server exl03 through the computer exlll. The coding processes may be performed by the camera exll6, the computer exlll, or the streaming server 6x103, or shared among them. [0253] Furthermore, the coding and decoding processes may be performed by an LSI ex500 generally included in each of the computer exlll and the devices. The LSI ex500 may be configured of a single chip or a plurality of chips. Software for coding and decoding video may be integrated into some type of a recording medium (such as a CD-ROM, a flexible disk, and a hard disk) that is readable by the computer exlll and others, and the coding and decoding processes may be performed using the software. Furthermore, when the cellular phone exll4 is equipped with a camera, the video data obtained by the camera may be transmitted. The video data is data coded by the LSI ex500 included in the cellular phone exll4. [0254] Furthermore, the streaming server exl03 may be composed of servers and computers, and may decentralize data and process the decentralized data, record, or distribute data. [0255] As described above, the clients may receive and reproduce the coded data in the content providing system exlOO. In other words, the clients can receive and decode information transmitted by the user, and reproduce the decoded data in real time in the content providing system exlOO, so that the user who does not have any particular right and equipment can implement personal broadcasting. [0256] Aside from the example of the content providing system exlOO, at least one of the moving picture coding apparatus (image coding apparatus) and the moving picture decoding apparatus (image decoding apparatus) described in each of embodiments mjay be implemented in a digital broadcasting system ex200 illustrated in FIG. 18. More specifically, a broadcast station ex201 communicates or transmits, via radio waves to a broadcast satellite ex202, multiplexed data obtained by multiplexing audio data and pthers onto video data. The video data is data coded by the moving picture coding method described in each of embodiments (i.e., data coded by the image coding apparatus according to an aspect of the present invention). Upon receipt of the multiplexed data, the broadcast satellite ex202 transmits radio waves for broadcasting. Then, a home-use antenna ex204 with a satellite broadcast reception function receives the radio waves. Next, a device such; as a television (receiver) ex300 and a set top box (STB) ex217 decodes the received multiplexed data, and reproduces the decoded data (i.e., functions as the image decoding apparatus according to an aspect of the present invention). [0257] Furthermore, a reader/recorder ex218 (i) reads and decodes the multiplexed data recorded on a recording medium ex215, such as a DVD and a BD, or (i) codes video signals in the recording medium ex215, and in some cases, writes data obtained by multiplexing an audio signal on the coded data. The reader/recorder ex218 can include the moving picture decoding apparatus or the moving picture coding apparatus as shown in each of embodiments. In this: case, the reproduced video signals are displayed on the monitor ex219, and can be reproduced by another device or system using the recording medium ex215 on which the multiplexed data is recorded. It is also possible to implement the moving picture decoding apparatus in the set top box ex217 connected to the cable ex2j03 for a cable television or to the antenna ex204 for satellite and/or terrestrial broadcasting, so as to display the video signals on the monitor ex219 of the television ex300. The moving picture decoding apparatus may be implemented not in the set top box:but in the television ex300. [0258] FIG. 19 illustrates the television (receiver) ex300 that uses the moving picture coding method and the moving picture decoding method described in each of embodiments. The television ex300 includes: a tuner ex301 that obtains or provides multiplexed data obtained by multiplexing audio data onto video data, through the antenna ex204 or the cable ex203, etc. that receives a broadcast; a modulation/demodulation unit ex302 that demodulates the received multiplexed data or modulates data into multiplexed data ;to be supplied outside; and a multiplexing/demultiplexing unit ex303 that demultiplexes the modulated multiplexed data into video data and audio data, or multiplexes video data and audio data coded by a signal processing unit ex306 into data. [0259] The television ex300 further includes: a signal processing unit ex306 including an audio signal processing unit ex304 and a; video signal processing unit ex305 that decode audio data and video data and code audio data and video data, respectively (which function as the image coding apparatus and the image decoding apparatus according to the aspects of the present invention); and an output unit ex309 including a speaker ex307 that provides the decoded audio signal, and a display unit ex308 that displays the decoded video signal, such as a display. Furthermore, the television 12x300 includes an interface unit ex317 including an operation input unit ex312 that receives an input of a user operation. Furthermore, the television ex300 includes a control unit ex310 that controls overall each constituent element of the television ex300, and a power supply circuit unit ex311 that supplies power to each of the elements. Other than the operation input unit ex312, the interface unit ex317 may include: a bridge ex313 that is connected to an external device, such as the reader/recorder ex218; a slot unit ex314 for enabling attachment of the recording medium ex216, such as an SD card; a driver ex315 to be connected to an external recording medium!, such as a hard disk; and a modem ex316 to be connected to a telephone network. Here, the recording medium ex216 can electrically record information using a non-volatile/volatile semiconductor memory element for storage. The constituent elements of the television ex300 are connected to each other through a synchronous bus. [0260] First, the configuration in which the television ex300 decodes multiplexed data obtained from outside through the antenna ex204 and others and reproduces the decoded data will be described. In the television ex300, upon a user operation through a remote controller ex220 and others, the multiplexing/demultiplexing unit ex303 demultiplexes the multiplexed data demodulated biy the modulation/demodulation unit ex302, under control of the control unit ex310 including a CPU. Furthermore, the audio signal processing unit ex304 decodes the demultiplexed audio data, and the video signal processing unit ex305 decodes the demultiplexed; video data, using the decoding method described in each of embodirhents, in the television ex300.The output unit ex309 provider the decoded video signal and audio signal outside, respectively. When the output unit ex309 provides the video signal and the audio signal, the signals may be temporarily stored in buffers ex318 and ex319, and others so that the signals are reproduced in synchronization with each other. Furthermore, the television ex300 may; read multiplexed data not through a broadcast and others but from the recording media ex215 and ex216, such as a magnetic disk, an optical disk, and a SD card. Next, a configuration in which the television ex300 codes an audio signal and a video signal, and transmits the data outside or writes the data on a recording medium will be described. In the television ex300, upon a user operation through the remote controller ex220 and others, the audio signal processing unit ex304 codes an audio signal, and the video signal processing unit ex305 codes a video signal, under control of the control unit ex310 using the coding method described in each of embodiments. The multiplexing /demultiplexing unit ex303 multiplexes the coded video signal and audio signal, and provides the resulting signal outside. When the multiplexing/demultiplexing unit ex303 multiplexes the video signal and the audio signal, the signals may be temporarily stored in the buffers ex320 and ex32i, and others so that the signals are reproduced in synchronization with each other. Here, the buffers ex318, ex319, ex320, and ex321 may be plural as illustrated, or at least one buffer may be shared pn the television ex300. Furthermore, data may be stored in a buffer so that the system overflow and underflow may be avoided between the modulation/demodulation unit ex302 and the multiplexing/demultiplexing unit ex303, for example. [0261] Furthermore, the television ex300 may include a configuration for receiving an AV input from a microphone or a camera othej- than the configuration for obtaining audio and video data from a broadcast or a recording medium, and may code the obtained data. Although the television ex300 can code, multiplex, and provide outside data in the description, it may be capable of only receiving, decoding, and providing outside data but not the coding, multiplexing^ and providing outside data. [0262] Furthermore, when the reader/recorder ex218 reads or Writes multiplexed data from or on a recording medium, one of the television ex300 and the reader/recorder ex218 may decode or code the multiplexed data, and the television ex300 and the reader/recorder ex218 may share the decoding or coding. [0263] As an example, FIG. 20 illustrates a configuration of an information reproducing/recording unit ex400 when data is read or written from or on an optical disk. The information reproducing/recording unit ex400 includes constituent elements ex401, ex402, ex403, ex404, ex405, ex406, and ex407 to be described hereinafter. The optical head ex401 irradiates a; laser spot in a recording surface of the recording medium ex215 that is an optical disk to write information, and detects reflected light from the recording surface of the recording medium ex215 to read the information. The modulation recording unit ex402 electrically drives a semiconductor laser included in the optical head ex401, and modulates the laser light according to recorded data, i The reproduction demodulating unit ex403 amplifies a reproduction signal obtained by electrically detecting the reflected light fnjm the recording surface using a photo detector included in the optical head ex401, and demodulates the reproduction signal by separating a signal component recorded on the recording medium ex2|15 to reproduce the necessary information. The buffer ex404 temporarily holds the information to be recorded on the recording medium iex215 and the information reproduced from the recording medium ex215. The disk motor ex405 rotates the recording medium ex215J The servo control unit ex406 moves the optical head ex401; to a predetermined information track while controlling the rotation drive of the disk motor ex405 so as to follow the laser spot. The system control unit ex407 controls overall the information reproducing/recording unit ex400. The reading and writing processes can be implemented by the system control unit ex407 using various information stored in the buffer ex404 and generating and adding new information as necessary, and by the modulation recording unit ex402, the reproduction demodulating unit ex403, and the servo control unit ex406 that record and reproduce information through the optical head ex401 while being operated in a coordinated manner. The system control unit ex407 includes, for examble, a microprocessor, and executes processing by causing a computer to execute a program for read and write. [0264] Although the optical head ex401 irradiates a laser spot in the description, it may perform high-density recording using near field light. [0265] FIG. 21 illustrates the recording medium ex215 that lis the optical disk. On the recording surface of the recording medium ex215, guide grooves are spirally formed, and an information; track ex230 records, in advance, address information indicating an absolute position on the disk according to change in a shape of the guide grooves. The address information includes information for determining positions of recording blocks ex231 that are a uhit for recording data. Reproducing the information track ex23G and reading the address information in an apparatus that records and reproduces data can lead to determination of the positions of the recording blocks. Furthermore, the recording medium ex215 includes a data recording area ex233, an inner circumference area ex232, and an outer circumference area ex234. The data recording area ex233 is an area for use in recording the user data. The'inner circumference area ex232 and the outer circumference area ex234 that are inside and outside of the data recording area ex233, respectively are for specific use except for recording the user data. The information reproducing/recording unit 400 reads and writes coded audio, coded video data, or multiplexed data obtained by multiplexing the coded audio and video data, from and on the data recording area ex233 of the recording medium ex215. [0266] Although an optical disk having a layer, such as a DVD;and a BD is described as an example in the description, the optical disk is not limited to such, and may be an optical disk having a multilayer structure and capable of being recorded on a part other than the surface. Furthermore, the optical disk may have a structure for multidimensional recording/reproduction, such as recording of information using light of colors with different wavelengths jn the same portion of the optical disk and for recording information having different layers from various angles. [0267] Furthermore, a car ex210 having an antenna ex205 can receive data from the satellite ex202 and others, and reproduce video on a display device such as a car navigation system ex211 set in the car ex210, in the digital broadcasting system ex200. Here, a configuration of the car navigation system ex211 will ; be a configuration, for example, including a GPS receiving unit frojn the configuration illustrated in FIG. 19. The same will be true for the configuration of the computer exlll, the cellular phone exll4, and others. [0268] FIG. 22A illustrates the cellular phone exll4 that uses the moving picture coding method and the moving picture decoding method described in embodiments. The cellular phone iexll4 includes: an antenna ex350 for transmitting and receiving; radio waves through the base station exllO; a camera unit ex365 capable of capturing moving and still images; and a display unit ex358 such as a liquid crystal display for displaying the data such as decoded video captured by the camera unit ex365 or received by the antenna ex350. The cellular phone exll4 further includes: a main body unit including an operation key unit ex366; an audio output unit 6x357 such as a speaker for output of audio; an audio input unit ex356 such as a microphone for input of audio; a memory unit ex367 for storing captured video or still pictures, recorded audio, coded or decoded data of the received video, the still pictures, e-mails, or others; and a slot unit ex364 that is an interface unit for a recording medium that stores data in the same manner as the memory unit ex367. [0269] Next, an example of a configuration of the cellular phone exll4 will be described with reference to FIG. 22B. In the cdlular phone exll4, a main control unit ex360 designed to control overall each unit of the main body including the display unit ex358 as well as the operation key unit ex366 is connected mutually, Via a synchronous bus ex370, to a power supply circuit unit ex361, an operation input control unit ex362, a video signal processing unit ex355, a camera interface unit ex363, a liquid crystal display (LCD) control unit ex359, a modulation/demodulation unit ex352, a multiplexing/demultiplexing unit ex353, an audio signal processing unit ex354, the slot unit ex364, and the memory unit ex367. i [0270] When a call-end key or a power key is turned ON by a user's operation, the power supply circuit unit ex361 supplies the respective units with power from a battery pack so as to activate the cell phone exll4. [0271] In the cellular phone exll4, the audio signal processing unit ex354 "converts the audio signals collected by the audio input unit ex356 in voice conversation mode into digital audio signals under the control of the main control unit ex360 including a CPU, ROM, anjd RAM. Then, the modulation/demodulation unit ex352 performs spread spectrum processing on the digital audio signals, an504, the driving frequency control unit ex512, the configuration of the control unit ex501 is not limited to such. For example, the signal processing unit ex507 may further include a CPU. Inclusion of another CPU in the signal processing unit ex507 can improve the processing speed. Furthermore, as another example, the CPU ex502 may serve as or be a part of the signal processing unit ex507, and, for example, may include an audio signal processing unit. In such a case, the control unit ex501 includes the signal processing unit ex507 or the CPU ex502 including a part of the signal processing unit ex507. ■ [0300] The name used here is LSI, but it may also be called IC, system LSI, super LSI, or ultra LSI depending on the degree of integration. [0301] Moreover, ways to achieve integration are not limited to the LSI, and a special circuit or a general purpose processor and so forth can also achieve the integration. Field Programmable GateiArray (FPGA) that can be programmed after manufacturing LSIs or a reconfigurable processor that allows re-configuration of the connection or configuration of an LSI can be used for the same purpose. [0302] In the future, with advancement in semiconductor technology, a brand-new technology may replace LSI. The functional blocks can be integrated using such a technology. The possibility is that the present invention is applied to biotechnology. [0303] [Embodiments] When video data generated in the moving picture coding method or by the moving picture coding apparatus described in each of embodiments is decoded, compared to when video data that conforms to a conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1 is decoded, the processing amount probably increases. Thus, the LSI ex500 needs to be set to a driving frequency higher than that of the CPU ex502 to be used when video data in conformity with the conventional standard is decoded. However, when the driving frequency is set higher, there is a problem that the power consumption increases. [0304] In order to solve the problem, the moving picture decoding apparatus, such as the television ex300 and the LSI ex500 is configured to determine to which standard the video data conforms, and switch between the driving frequencies according to the determined standard. FIG. 32 illustrates a configuration ex800 in the present embodiment. A driving frequency switching unit isx803 sets a driving frequency to a higher driving frequency when:video data is generated by the moving picture coding method or the moving picture coding apparatus described in each of embodiments. Then, the driving frequency switching unit ex803 instructs a decoding processing unit ex801 that executes the moving picture decoding method described in each of embodiments to decode the video data. When the video data conforms to the conventional standard, the driving frequency switching unit ex803 sets a driving frequency to a lower driving frequency than that of the video data generated by the moving picture coding method or the moving picture coding apparatus described in each of embodiments. Then, the driving frequency switching unit ex803 instructs the decoding processing unit ex802 that conforms to the conventional standard to decode the video data. [0305] More specifically, the driving frequency switching unit ex803 includes the CPU ex502 and the driving frequency control unit ex512 in FIG. 31. Here, each of the decoding processing unit ex801 that executes the moving picture decoding method described in each of embodiments and the decoding processing unit ex802 that conforms to the conventional standard corresponds to the signal processing unit ex507 in FIG. 31. The CPU ex502 determines to which staindard the video data conforms. Then, the driving frequency control unit ex512 determines a driving frequency based on a signal from trie CPU ex502. Furthermore, the signal processing unit ex507 decodes the video data based on the signal from the CPU ex502. For example, the identification information described in Embodiment 6 is probably used for identifying the video data. The identification information is not limited to the one described in Embodiment 6 but may be any information as long as the information indicates to which standard the video data conforms. For example, when which standard; video data conforms to can be determined based on an external signal for determining that the video data is used for a television or a disk, etc., the determination may be made based on such an external signal. Furthermore, the CPU ex502 selects a driving frequency based on, for example, a look-up table in which the standards of the video data are associated with the driving frequencies as shown in FIG. 34. The driving frequency can be selected by storing the look-up table in the buffer ex508 and in an internal memory of an LSI, and with reference to the look-up table by the CPU ex502. -7tr [0306] FIG. 33 illustrates steps for executing a method in the present embodiment. First, in Step exS200, the signal processing unit ex507 obtains identification information from the multiplexed; data. Next, in Step exS201, the CPU ex502 determines whether or not the video data is generated by the coding method and the coding apparatus described in each of embodiments, based oh the identification information. When the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, in Step exS202, the CPU ex502 transmits a signal for setting the driving frequency to a higher driving frequency to the driving frequency control unit ex512. Then, the driving frequency control unit ex512 sets the driving frequency to the higher driving frequency. On the other hand, when the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1, in Step exS203, the CPU ex502 transmits a signal for sietting the driving frequency to a lower driving frequency to the driving frequency control unit ex512. Then, the driving frequency control unit ex512 sets the driving frequency to the lower driving frequency than that in the case where the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiment. [0307] Furthermore, along with the switching of the driving frequencies, the power conservation effect can be improved by changing the voltage to be applied to the LSI ex500 or an apparatus including the LSI ex500. For example, when the driving frequency is set lower, the voltage to be applied to the LSI ex500 or the apparatus including the LSI ex500 is probably set to a voltage lower than that in the case where the driving frequency is set higher. [0308] Furthermore, when the processing amount for decoding is larger, the driving frequency may be set higher, and when the processing amount for decoding is smaller, the driving frequency may be set lower as the method for setting the driving frequency. ;Thus, the setting method is not limited to the ones described above. For example, when the processing amount for decoding video data in conformity with MPEG-4 AVC is larger than the processing amount for decoding video data generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the driving frequency is probably set in reversejorder to the setting described above. [0309] Furthermore, the method for setting the driving frequency is not limited to the method for setting the driving frequency lower. For example, when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the voltage to be applied to the LSI ex500 or the apparatus including the LSI ex500 is probably set higher. When the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-|l, the voltage to be applied to the LSI ex500 or the apparatus including the LSI ex500 is probably set lower. As another example, when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, the driving of the CPU ex502 does not probably have to be suspended. When the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and;VC-l, the driving of the CPU ex502 is probably suspended at a given time because the CPU ex502 has extra processing capacity. Even; when the identification information indicates that the video data is generated by the moving picture coding method and the moving picture coding apparatus described in each of embodiments, |in the case where the CPU ex502 has extra processing capacity, the driving of the CPU ex502 is probably suspended at a given time. In such a case, the suspending time is probably set shorter than that m the case where when the identification information indicates that the video data conforms to the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1. [0310] Accordingly, the power conservation effect can be improved by switching between the driving frequencies in accordance with the standard to which the video data conforms. Furthermore, when the LSI ex500 or the apparatus including the LSI ex500 is driven using a battery, the battery life can be extended with the power conservation effect. [0311] [Embodiment 9] There are cases where a plurality of video data that conforms to different standards, is provided to the devices and systemsj, such as a television and a cellular phone. In order to enable decoding the plurality of video data that conforms to the different standards, the signal processing unit ex507 of the LSI ex500 needs to conform to the different standards. However, the problems of increase iin the scale of the circuit of the LSI ex500 and increase in the cost arise with the individual use of the signal processing units ex507 that conform to the respective standards. [0312] In order to solve the problem, what is conceived; is a configuration in which the decoding processing unit for implementing the moving picture decoding method described in each of embodiments and the decoding, processing unit that conforms ito the conventional standard, such as MPEG-2, MPEG-4 AVC, and VC-1 are partly shared. Ex900 in FIG. 35A shows an example of the configuration. For example, the moving picture decoding method described in each of embodiments and the moving picture decoding method that conforms to MPEG-4 AVC have, partly in common, the details of processing, such as entropy coding, inverse quantization, deblocking filtering, and motion compensated prediction.; The details of processing to be shared probably include use of a decoding processing unit ex902 that conforms to MPEG-4 AVC. In contrast, a dedicated decoding processing unit ex901 is probably used for other processing unique to an aspect of the present invention. Since the aspect of the present invention is characterized by inter prediction in particular, for example, the dedicated decoding processing unit ex901 is used for inter prediction. Otherwise, the decoding processing unit is probably shared for one of the entropy decoding, deblocking filtering, and inverse quantization, or all of the processing. The decoding processing unit for implementing the moving picture decoding method described in each of embodiments may be shared for the processing to be shared, and a dedicated decoding processing unit may be used for processing unique to that of MPEG-4 AVC. [0313] Furthermore, exlOOO in FIG. 35B shows another example in that processing is partly shared. This example uses a configuration including a dedicated decoding processing unit exlOOl that supports the processing unique to an aspect of the present invention, a dedicated decoding processing unit exl002 that supports the processing unique to another conventional standard, and a decoding processing unit exl003 that supports processing to be shared between the moving picture decoding method according to the aspect of the present invention and the conventional moving picture decoding method. Here, the dedicated decoding processing! units exlOOl and exl002 are not necessarily specialized for the processing according to the aspect of the present invention and the processing of the conventional standard, respectively, and may be the ones capable of implementing general processing. Furthermore, the configuration of the present embodiment can be implemented by the LSI ex500. [0314] As such, reducing the scale of the circuit of an LSI and reducing the cost are possible by sharing the decoding processing unit for the processing to be shared between the moving picture decoding method according to the aspect of the present invention and the moving picture decoding method in conformity with the conventional standard. [0315] [Industrial Applicability] The present invention is applicable to a television receiver, a digital video recorder, a car navigation system, a cellular phone, a digital camera, a digital video camera, and the like. [Reference Signs List] [0316] 500,1300 Image coding apparatus 501 Subtracting unit 502 Transforming unit 503 Quantizing unit 504 Entropy coder 505, 602 Inverse quantizing unit 506, 603 Inverse transforming unit 507, 604 Adding unit 508,605 Block memory 509, 606 Picture memory 510.607 Intra predicting unit 511.607 Inter predicting unit 512, 609, 1303, 1403 Selecting unit 600,1400 Image decoding apparatus 601 Entropy decoder 1301,1401 Deriving unit 1302, 1402 Adding unit 1304 Coder 1404 Decoder CLAIMS [Claim 1] An image encoding method of encoding each block among blocks of pictures, the image encoding method comprising: deriving a candidate for a motion vector of a current block to be encoded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; adding the derived candidate to a list of candidates; selecting one motion vector from the list of candidates; and encoding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture; deriving the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 2] The image encoding method according to Claim 1, wherein, in the deriving: the deriving of the candidate from the first motion vector of the first block is not performed in the case of determining that one of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture and the other one of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture; and the deriving of the candidate from the first motion vector of the first block is performed in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture or in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 3] The image encoding method according to one of Claim 1 and Claim 2, wherein the encoding further includes encoding information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and information indicating whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture. [Claim 4] The image encoding method according to any one of Claim 1 to Claim 3, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, using a temporal distance between the reference picture of the current block and the picture that includes the current block; and determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, using a temporal distance between the first reference picture of the first block and the first picture that includes the first block. [Claim 5] The image encoding method according to any one of Claim 1 to Claim 4, wherein the deriving includes determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, in a period during which the first block is encoded. [Claim 6] The image encoding method according to any one of Claim 1 to Claim 4, wherein the deriving includes determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, in a period during which the current block is encoded. [Claim 7] The image encoding method according to any one of Claim 1 to Claim 6, wherein the deriving includes: deriving the first motion vector of the first block as the candidate, in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate by scaling the first motion vector of the first block using a ratio, in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture, the ratio being a ratio of a temporal distance between the reference picture of the current block and the picture that includes the current block to a temporal distance between the first reference picture of the first block and the first picture that includes the first block. [Claim 8] The image encoding method according to any one of Claim 1 to Claim 7, wherein the deriving further includes, without deriving the candidate from the first block, selecting another first block and deriving the candidate from a motion vector of the other first block in the case of determining that the reference picture of the current block is a short-term reference picture and the first reference picture of the first block is a long-term reference picture, the other first block being encoded with reference to a short-term reference picture. [Claim 9] An image decoding method of decoding each block among blocks of pictures, the image decoding method comprising: deriving a candidate for a motion vector of a current block to be decoded, from a first motion vector of a first block Included in a first picture, the first picture being different from a picture that includes the current block; adding the derived candidate to a list of candidates; selecting one motion vector from the list of candidates; and decoding the current block using the selected motion vector and a reference picture of the current block, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture; deriving the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 10] The image decoding method according to Claim 9, wherein, in the deriving: the deriving of the candidate from the first motion vector of the first block is not performed in the case of determining that one of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture and the other one of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture; and the deriving of the candidate from the first motion vector of the first block is performed in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture or in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 11] The image decoding method according to one of Claim 9 and Claim 10, wherein the decoding further includes decoding information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and information indicating whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, and the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, using the information indicating whether the reference picture of the current block is a long-term reference picture or a short-term reference picture; and determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, using the information indicating whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture. [Claim 12] The image decoding method according to one of Claim 9 and Claim 10, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, using a temporal distance between the reference picture of the current block and the picture that includes the current block; and determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, using a temporal distance between the first reference picture of the first block and the first picture that includes the first block. [Claim 13]The image decoding method according to any one of Claim 9 to Claim 12, wherein the deriving includes determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, in a period during which the first block is decoded. [Claim 14] The image decoding method according to any one of Claim 9 to Claim 12, wherein the deriving includes determining whether the first reference picture of the first block is a long-term reference picture or a short-term reference picture, in a period during which the current block is decoded. [Claim 15] The image decoding method according to any one of Claim 9 to Claim 14, wherein the deriving includes: deriving the first motion vector of the first block as the candidate, in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate by scaling the first motion vector of the first block using a ratio, in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture, the ratio being a ratio of a temporal distance between the reference picture of the current block and the picture that includes the current block to a temporal distance between the first reference picture of the first block and the first picture that includes the first block. [Claim 16] The image decoding method according to any one of Claim 9 to Claim 15, wherein the deriving further includes, without deriving the candidate from the first block, selecting another first block and deriving the candidate from a motion vector of the other first block in the case of determining that the reference picture of the current block is a short-term reference picture and the first reference picture of the first block is a long-term reference picture, the other first block being decoded with reference to a short-term reference picture. [Claim 17] An image encoding apparatus that encodes each block among blocks of pictures, the image encoding apparatus comprising: a deriving unit configured to derive a candidate for a motion vector of a current block to be encoded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; an adding unit configured to add the derived candidate to a list of candidates; a selecting unit configured to select one motion vector from the list of candidates; and an encoder configured to encode the current block using the selected motion vector and a reference picture of the current block, wherein the deriving unit is configured to: determine whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture; derive the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and derive the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 18] An image decoding apparatus that decodes each block among blocks of pictures, the image decoding apparatus comprising: a deriving unit configured to derive a candidate for a motion vector of a current block to be decoded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; an adding unit configured to add the derived candidate to a list of candidates; a selecting unit configured to select one motion vector from the list of candidates; and a decoder configured to decode the current block using the selected motion vector and a reference picture of the current block, wherein the deriving unit is configured to: determine whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture derive the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and derive the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 19] An image encoding and decoding apparatus comprising: the image encoding apparatus according to Claim 17; and an image decoding apparatus that decodes each block among blocks of pictures, wherein the image decoding apparatus includes: a deriving unit configured to derive a candidate for a motion vector of a current block to be decoded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; an adding unit configured to add the derived candidate to a list of candidates; a selecting unit configured to select one motion vector from the list of candidates; and a decoder configured to decode the current block using the selected motion vector and a reference picture of the current block, wherein the deriving unit is configured to: determine whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture; derive the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and derive the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 20] A content providing method of transmitting, from a server in which image data encoded by the image encoding method according to Claim 1 is recorded, to an external terminal, the image data in response to a request from the external terminal. [Claim 21] The image decoding method according to any one of Claim 9 to Claim 16, further comprising: switching between decoding that conforms to a first standard and decoding that conforms to a second standard according to an identifier indicating one of the first standard and the second standard, the identifier being included in an encoded bit-stream, wherein when the identifier indicates the first standard, the deriving is performed. [Claim 22] A motion vector deriving method for deriving a motion vector for each block among blocks of pictures, the motion vector deriving method comprising: deriving a candidate for a motion vector of a current block to be coded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; adding the derived candidate to a list of candidates; and selecting one motion vector from the list of candidates, wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture; deriving the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture. [Claim 23] Control circuitry used for an image processing system, the image processing system for decoding each block among blocks of pictures, the image processing system includes storage accessible from the control circuitry, the control circuitry configured using the storage to perform operations comprising: deriving a candidate for a motion vector of a current block to be coded, from a first motion vector of a first block included in a first picture, the first picture being different from a picture that includes the current block; adding the derived candidate to a list of candidates; and selecting one motion vector from the list of candidates; wherein the deriving includes: determining whether the reference picture of the current block is a long-term reference picture or a short-term reference picture, and whether a first reference picture of the first block is a long-term reference picture or a short-term reference picture deriving the candidate from the first motion vector without scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a long-term reference picture; and deriving the candidate from the first motion vector by scaling based on a temporal distance in the case of determining that each of the reference picture of the current block and the first reference picture of the first block is a short-term reference picture.