BACKGROUND OF THE INVENTION The present invention relates to devices, methods, and control programs used to control rolling operations and, more particularly, to shape control of rolled materials. It is important for quality assurance to roll a material into a product having neither center wave nor edge waves. Therefore, in recent years, it has become customary to make uniform the thicknesswise dimension of each rolled material along its rolling direction. Moreover, it has become customary to provide rolling control while taking account of the thickness distribution across each material, i.e., in the width direction (that is, to effect shape control of rolled materials). To date, various methods have been proposed. One example of such method is to perform a rolling operation repeatedly using plural passes. In this case, there is a proposed method of controlling work roll bending force based on shape data and strip crown information obtained by a measurement during or after a rolling operation in one pass (see, for example, JP-A-3-32412). Furthermore, a method of shape control including two procedures is proposed, for example, in JP-A-9-295022. The first procedure consists of continuously measuring the shape of a rolled material during rolling and successively determining the work roll bending force so as to correct poor shape of the material being rolledfi-om continuously obtained data and from the prediction rate of variation of strip crown derived by a prediction model of strip crown variation rates. The second procedure consists of successively modifying the prediction model of strip crown variation rates from the continuously obtained data and from the predicted strip crown variation rate of the prediction model. Where a rolling operation is performed while repeating plural passes, forward rolling and reverse rolling steps are repeatedly performed. During the forward rolling step, a material to be rolled that is wound around a reel is wound off The rolling mill is rotated in a certain direction to perform rolling. The rolled material is taken up on a reel mounted opposite to the rolling mill. During the reverse rolling step, the wound rolled material is wound off, the rolling mill is rotated in reverse, and rolling is effected. Thus, rolling is carried out in plural passes, i.e., reverse rolling. In such reverse rolling, if the method disclosed in the above-cited JP-A-3-32412 is used, it is necessary to mount a shape meter on each side of the rolling mill, the shape meter operating to measure the shape of the rolled material at plural positions spaced apart across the width of the rolled material. However, such shape meters are expensive and account for nonnegligible percentages of the initial investment cost and maintenance cost of the rolling mill. That is, considerable cost reduction effects can be anticipated if the shape meters mounted on both sides of the rolling mill can be reduced to one shape meter on one side of the mill. SUMMARY OF THE INVENTION In view of the foregoing circumstances, it is an object of the present invention to provide a rolling mill which can perform reverse rolling and which can effect shape control of a rolled material even if a shape meter for measuring the shape of the rolled material is mounted on only one side of the mill. One embodiment of the present invention is a rolling control device adapted to control a rolling mill that performs plural rolling operations by alternately and repeatedly performing a first rolling operation in which a rolled material in the form of a strip or plate is conveyed, held between at least one pair of rolls so as to be rolled, and then taken up on a coiler and a second rolling operation in which the rolls are rotated in reverse, the material in wound state is wound off, conveyed in a direction opposite to the direction in which the material was conveyed in a previous rolling operation such that the material is rolled, and then taken up on a coiler. The rolling mill has a shape meter for measuring the shape of the conveyed rolled material at plural positions spaced apart in a width direction of the material which is perpendicular to the direction of conveyance of the rolled material and which is parallel to a strip surface of the rolled material, the shape meter being mounted on only one side of the rolls relative to the direction of conveyance of the material. The rolling control device includes: a shape comparison result calculating unit for calculating a shape comparison result being a result of a comparison between the shape of the rolled material and a target value of the shape of the rolled material; a controlled variable decision unit for determining controlled variables used to control the rolls so as to correct the difference between the shape of the rolled material and the target value based on the calculated shape comparison result; and a shape prediction unit for calculating a prediction value of the shape at the plural positions spaced apart in the width direction of the rolled material based on an entrance side strip thickness being a strip thickness of the rolled material obtained before the material is rolled by the rolls, on an exit side strip thickness being a strip thickness of the material obtained after it has been rolled, on an average value of the entrance side strip thickness, and on an average value of the exit side strip thickness. In the first rolling operation, the shape comparison result calculating unit calculates the shape comparison resuh by comparing the shape of the rolled material measured by the shape meter with the target value. In the second roIHng operation, the shape comparison result calculating unit calculates the shape comparison result by comparing the prediction value of the shape calculated by the shape prediction unit with the target value. Another embodiment of the invention provides a rolling control method for controlling a rolling mill that performs plural rolling operations by alternately and repeatedly performing a first rolling operation in which a rolled material in the form of a strip or plate is conveyed, held between at least one pair of rolls so as to be rolled, and then taken up on a coiler and a second rolling operation in which the rolls are rotated in reverse, the material in wound state is wound off, conveyed in a direction opposite to the direction in which the material was conveyed in a previous rolling operation such that the material is rolled, and then taken up on a coiler. The rolling mill has a shape meter for measuring the shape of the conveyed rolled material at plural positions spaced apart in a width direction of the material which is perpendicular to the direction of conveyance of the rolled material and which is parallel to a strip surface of the rolled material, the shape meter being mounted on only one side of the rolls relative to the direction of conveyance of the material. The rolling control method starts with calculating prediction values of the shape at the plural positions spaced apart in the width direction of the rolled material, based on an entrance side strip thickness being a strip thickness of the material obtained before the material is rolled by the rolls, on an exit side strip thickness being a strip thickness of the material obtained after it has been rolled, on an average value of the entrance side strip thickness, and on an average value of the exit side strip thickness. In the first rolling operation, the shape of the rolled material measured by the shape meter is compared against the target value to thereby calculate a shape comparison result being a result of a comparison between the shape of the rolled material and the target value. In the second rolling operation, the shape comparison result is calculated by comparing the calculated prediction value of the shape and the target value. Controlled variables for controlling the rolls are determined such that the difference between the shape of the rolled material and the target value is corrected based on the calculated shape comparison result. A further embodiment of the invention provides a rolling control program for controlling a rolling mill that performs plural rolling operations by alternately and repeatedly performing a first rolling operation in which a rolled material in the form of a strip or plate is conveyed, held between at least one pair of rolls so as to be rolled, and then taken up on a coiler and a second rolling operation in which the rolls are rotated in reverse, the rolled material in wound state is wound off, conveyed in a direction opposite to the direction in which the material was conveyed in a previous rolling operation to roll the material, and then taken up on a coiler. The rolling mill has a shape meter for measuring the shape of the conveyed rolled material at plural positions spaced apart in a width direction of the material which is perpendicular to the direction of conveyance of the rolled material and which is parallel to a strip surface of the material, the shape meter being mounted on only one side of the rolls relative to the direction of conveyance of the material. The rolling control program causes an information processor to execute the steps of calculating prediction values of the shape at the plural positions spaced apart in the width direction of the rolled material, based on an entrance side strip thickness being a strip thickness of the material obtained before the material is rolled by the rolls, on an exit side strip thickness being a strip thickness of the material obtained after it has been rolled, on an average value of the entrance side strip thickness, and on an average value of the exit side strip thickness; comparing the shape of the rolled material measured by the shape meter and a target value to thereby calculate a shape comparison result being a result of a comparison between the shape of the rolled material and the target value in the first rolling operation; calculating the shape comparison result by comparing each calculated prediction value of the shape and the target value in the second rolling operation; and determining controlled variables for controlling the rolls so as to correct the difference between the shape of the rolled material and the target value based on the calculated shape comparison result. Use of the present invention makes it possible to effect shape control of a rolled material even where a shape meter for measuring the shape of the rolled material is mounted on only one side of a rolling mill that performs reverse rolling. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing the whole configuration of a rolling mill associated with one embodiment of the present invention. FIGS. 2A and 2B show flexure of work rolls and the shape of a rolled material. FIG. 3 A and 3B show the shape of a rolled material before and after rolling. FIG. 4 is a block diagram of a rolling control device associated with one embodiment of the present invention, showing the hardware configuration of the device. FIG. 5 is a diagram showing the configuration of a shape prediction modeling module associated with one embodiment of the invention. FIG. 6 is a flowchart illustrating operations performed when a rolling mill associated with one embodiment of the invention rotates forwardly. FIG. 7 is a flowchart illustrating operations performed when the rolling mill associated with one embodiment of the invention rotates reversely. -5¬FIG. 8 is a diagram showing the configuration of a strip shape prediction modeling module associated with another embodiment of the invention. FIG. 9 is a diagram illustrating the whole configuration of a rolling mill associated with a further embodiment of the invention. DETAILED DESCRIPTION OF THE INVENTION Embodiments of the present invention are hereinafter described in detail with reference to the drawings. FIG 1 is a diagram showing the whole configuration of a rolling mill 1 according to one embodiment of the present invention. The rolling mill 1 includes work rolls 101a, 101b (hereinafter may be referred to as the work rolls 101) for rolling a material A to be rolled by making contact with the rolled material A, back up rolls 102a, 102b (hereinafter may be referred to as the back up rolls 102) creating a rolling load (rolling force), and intermediate rolls 103a, 103b (hereinafter may be referred to as the intermediate rolls 103) mounted between the work rolls 101 and back up rolls 102. In the following description, the work rolls 101, back up rolls 102, and intermediate rolls 103 are collectively referred to as a rolling mill. The rolling mill associated with the present embodiment carries out reverse rolling for machining the rolled material A to a desired thickness by plural rolling operations, by alternately repeating forward-rotation rolling and reverse-rotation rolling. In the forward-rotation rolling, each roll is rotated in one direction, and the material is rolled while being transported. In the reverse-rotation rolling, each roll is rotated in reverse, and the material is rolled while being conveyed. Roll benders (not shown) for adjusting the pressure between the rolls are connected to the work rolls 101 and intermediate rolls 103. A work roll bender control device 104 and an intermediate roll bender control device 105 control the pressure exerted by the roll benders, respectively, under control of a rolling control device 110. A draft control device 106 is connected to the back up rolls 102 and applies a rolling load to the back up rolls 102 under control of the rolling control device 110. A rolling mill load cell 108 measures the rolling load on the back up rolls 102 and enters the measured load to the rolling control device 110. A shape meter 107 is mounted on one side of the direction in which the material A is rolled and conveyed by the rolling mill. The shape meter 107 measures the shape of the strip surface of the rolled material A at plural positions spaced apart in the width direction of the material A which is perpendicular to the direction of conveyance and which is parallel to the strip surface of the rolled material A, and enters the measured shape to the rolling control device 110. Thus, the control device 110 acquires data by actual measurement of the shape of the rolled material A. The data obtained by the actual measurement is used by the rolling control device 110 to control the roll benders (i.e., shape control) by means of the work roll bender control device 104 and intermediate roll bender control device 105. In the present embodiment, the shape meter 107 is mounted only to the right of the rolling mill as viewed in FIG. 1, i.e., only the side in which the already rolled material A is conveyed during forward rotation. Accordingly, only during forward rotation, data can be obtained by actual measurement of the shape of the material A already rolled. The shape of the rolled material A actually measured by the shape meter 107 is now described. Since the work rolls 101 are so mounted that both longitudinal sides of the work rolls 101 rolling the material A are supported, if the rolled material A is sandwiched between the upper and lower work rolls 101, the rolls will suffer from flexure. FIG. 2A shows flexure of the work rolls 101 between which the rolled material A is held. FIG. 2 A shows the state of the work rolls 101 as viewed from the direction of conveyance of the rolled material A. When the material is rolled while the work rolls 101 are sagging as shown in FIG. 2A, the material A that has undergone a rolling operation has a varied thickness in a direction which is parallel to the strip surface of the material and is vertical to (hereinafter also referred to as the width direction) the direction of conveyance as shown in FIG. 2B. That is, the strip thickness at both ends in the width direction is here designated as hmin and minimal. The strip thickness at the center in the width direction is designated as hmax and maximal. Let hi be the average value of the thickness distribution in the width direction. This distribution of strip thicknesses in the width direction is a strip thickness profile. FIGS. 3 A and 3B illustrate the concept of data e(i) about a shape measured by the shape meter 107 associated with the present embodiment. When the rolled material A sags as shown in FIG. 2B, the length of the strip material is increased in the direction of conveyance because both ends in the width direction are rolled to a smaller dimension than the middle of the material provided that the width of the rolled material does not vary before and after the rolling operation. FIG. 3 A shows the state of the material A yet to be rolled, as viewed from the direction vertical to the strip surface of the material. FIG. 3B shows the state of the already rolled material A, as viewed from the direction vertical to the strip surface of the material. Where the average value of the length of the material A that has been rolled is set to 1, the length of the rolled material A taken at a position corresponding to a factor "i" indicating the width direction is represented by "1 + 8(i)" or "1 - S(i)", where 8(i) is the absolute value of the deviation from the average value as shown in FIG. 3B. The shape data S(i) defined in this way is obtained as actual measurement values by the shape meter 107 associated with the present embodiment. In the rolling mill 1 associated with the present embodiment, the aforementioned shape control improves the uniformity of the widthwise thickness of the rolled material A as well as the uniformity of the thickness taken in the direction of conveyance of the rolled material A. Therefore, the shape meter 107 would normally be mounted on each side of the direction of rolling performed by the rolling mill. In the present embodiment, the shape meter 107 is mounted on only on one side but shape control is permitted during forward rotation and during reverse rotation. The provision of only one shape meter 107 can reduce the cost of the mill, constituting the feature of the present embodiment. As shown in FIG. 1, the rolling control device 110 includes a shape prediction modeling module 111, a shape deviation calculating portion 112, a target shape setting portion 113, and a controlled variable decision portion 114. The modeling module 111 is configured in association with the feature of the present embodiment. When a reverse rolling operation is performed as shown in FIG. 1, i.e., the shape meter 107 is not mounted on the side into which the material A already rolled is conveyed, prediction shape data that are prediction values of the shape of the already rolled material A are generated by computations using the shape prediction model and entered into the shape deviation calculating portion 112. That is, the shape prediction modeling module 111 functions as a controlled variable decision portion that predicts the shape of the already rolled material A. Details of the shape prediction modeling module 111 will be described later. The shape deviation calculating portion 112 calculates a shape deviation based on actual shape data obtained from the shape meter 107 and on shape data (hereinafter may be referred to as the target shape data) giving a target and set by the target shape setting portion 113 when the rolling mill is rotating forwardly. The shape deviation calculating portion 112 calculates a shape deviation based on prediction shape data enteredfrom the shape prediction modeling module 111 and on the target shape data set by the target shape setting portion 113 during reverse rotation of the mill. In particular, the shape deviation calculating portion 112 acts as a shape comparison result calculating portion for computationally producing a shape comparison result that is the result of a comparison between an actual measurement value or predictive value of the shape of the rolled material A and a target value of the shape of the rolled material A. The shape deviation calculating portion 112 enters the calculated shape deviation into the controlled variable decision portion 114. Data about the target shape values either previously stored or entered by a user are output as target shape data from the target shape setting portion 113 to the shape deviation calculating portion 112. The controlled variable decision portion 114 determines and outputs controlled variables for the roll benders and back up rolls provided by the work roll bender control device 104, intermediate roll bender control device 105, and draft control device 106 based on the shape deviation entered from the shape deviation calculating portion 112. Thus, the work roll bender control device 104, intermediate roll bender control device 105, and draft control device 106 control the roll benders and back up rolls. The controlled variable decision portion 114 determines the controlled variables to correct the shape deviation computed as described above, i.e., the difference between the shape of the rolled material and the target value. Furthermore, the decision portion 114 enters the actual values of the controlled variables provided by the work roll bender control device 104, intermediate roll bender control device 105, and draft control device 106 into the shape prediction modeling module 111. Because of the configuration described so far, the rolling control device 110 effects shape control of the rolled material A based on the actual shape data obtained by the shape meter 107 or on the shape data predicted by the shape prediction modeling module. The rolling control device 110 is accomplished by an information processor such as a personal computer. The hardware configuration of the rolling control device 110 associated with the present embodiment is now described by referring to FIG. 4. The rolling control device 110 associated with the present embodiment includes constituent components similar to those of a general server or personal computer as shown in FIG. 4. Specifically, the information processor associated with the present embodiment has a CPU (central processing unit) 10, a RAM (random access memory) 20, a ROM (read only memory) 30, an HDD (hard disk drive) 40, and an I/F 50 which are interconnected via a bus 80. An LCD (liquid crystal display) 60 and a manual control portion 70 are connected with the I/F 50. The CPU 10 is an arithmetic means and controls the whole operation of the rolling control device 110. The RAM 20 is a volatile storage medium capable of reading and writing information at high speed. When information is processed by the CPU 10, the RAM is used as a working area. The ROM 30 is a read-only nonvolatile storage medium and loaded with computer programs such as firmware. The HDD 40 is a nonvolatile storage medium capable of reading and writing information and stores an OS (operating system), various control programs, application programs, and so on. The I/F 50 connects the bus 80 with various pieces of hardware, a network, and so on and controls them. The LCD 60 is a visual user interface permitting a user to check the state of a distribution server (not shown). The manual control portion 70 is a user interface (such as a keyboard, mouse, touch panel, or the like) permitting a user to enter information into the rolling control device 110. In this hardware configuration, a program stored in the ROM 30, HDD 40, or other storage medium (such as an optical disk) (not shown) is read into the RAM 20 and executed under control of the CPU 10, thus constituting a software control portion. A combination of the software control portion configured in this way and hardware constitutes a fiinctional block that implements the fiinctions of the rolling control device 110 associated with the present embodiment. In FIG. 4, a case in which the rolling control device 110 is formed by the single information processor is described as one example. The fiinctions of the rolling control device 110 may be implemented as shown in FIG. 1 by performing processing by cooperative operation of plural information processors. The fiinctions of the shape prediction modeling module 111 associated with the present embodiment are next described. FIG. 5 is a diagram illustrating the fixnctions incorporated in the shape prediction modeling module 111 and the cooperative operation of the various fiinctions. As shown in FIG. 5, the shape prediction modeling module 111 includes three computational submodules, i.e., a roll gap model Ilia, an exit side strip thickness profile model 111b, and a shape prediction model 111c. The roll gap model 111a is a computational module for calculating a roll gap between the work rolls 101, based on parameters measured when the rolling mill is controlled by the work roll bender control device 104, the intermediate roll bender control device 105, and the draft control device 106. The model Ilia computes the roll gap S(i) using the following Eq. (1); where Fw is the bender pressure on the work rolls 101 and can be obtained via the work roll bender control device 104, Fi is the bender pressure on the intermediate rolls 103 and can be obtained via the intermediate roll bender control device 105, Pps, Pws are the rolling loads on the back up rolls 102 and occurring on the opposite sides of the axis of the rolls, AP is the amount of variation of the rolling load (rolling force) and can be obtained via the rolling mill load cell 108, and ASgap is the amount of variation of the roll gap and can be obtained by a dedicated sensor (not shown). The "i" included in the roll gap S(i) is a factor indicating a position taken in the width direction of the rolled material A as already described in connection with FIG. 3. The exit side strip thickness profile model 111b is a computational module for calculating the exit side strip thickness h(i) of the material A already rolled by the rolling mill and computes the exit side strip thickness h(i) using Eq. (2). where M is the mill modulus of the rolling mill and a preset value is used as the mill modulus, P(i) is the rolling load responsive to the widthwise position of the rolled material A and found based on PDS and Pws, and a calculation result obtained using the roll gap model 11 la is used as S(i). The shape prediction model 111 c is a calculational module for calculating the prediction shape data X(i) responsive to the aforementioned shape data S(i) obtained by actual measurement during a rolling operation using reverse rotation of the rolling mill by the use of Eq. (3). 2 -1 ^(^> ^1 where h(i) is the exit side strip thickness calculated by the exit side strip thickness profile model and H(i) is the entrance side strip thickness profile indicating the strip thickness obtained before the material A is rolled by the rolling mill. As described previously, reverse rolling is presumed in the present embodiment. Therefore, the shape prediction modeling module 111 finds the entrance side strip thickness profile H(i) in the current pass by reversing and converting the time sequential values of the exit side strip thickness h(i) calculated by the exit side strip thickness profile model 111b during the rolling operation in the previous pass. Hi is the average value of the entrance side strip thickness H(i). hi is the average value of the exit side strip thickness h(i) as already described in connection with FIG. 2B. These are found using Eqs. (4) and (5), respectively. where the entrance side strip thickness H(i) is found by converting the already computed exit side strip thickness in the previous pass as described previously. Accordingly, the shape prediction modeling module 111 can previously find the entrance side strip thickness average value Hi by executing a calculational operation given by Eq. (4) when a rolling operation in the current pass is started. On the other hand, the exit side strip thickness average value hi is successively recalculated based on the exit side strip thickness h(i) that is successively calculated using Eq. (2). That is, the shape prediction modeling module 111 recalculates the exit side strip thickness average value hi by executing the calculational operation given by Eq. (5) in response to the computation of the exit side strip thickness h(i). In this way, even when a rolling operation is performed using reverse rotation of the rolling mill and the shape meter 107 is not mounted on the exit side of the mill, shape data about the rolled material A can be obtained as prediction shape data X(i) owing to the result of the calculation performed by the shape prediction model 111c. The feature of the present embodiment is that the aforementioned shape control is effected using the prediction shape data Furthermore, the shape prediction model 111c estimates the strip thickness of the material A yet to be rolled (i.e., the entrance side strip thickness) by the rolling mill using Eq. (6) and the actual shape data S(i) obtained by the shape meter 107 during a rolling operation using forward rotation of the mill. Eq. (6) above has been obtained by taking the prediction shape data X(i) in Eq. (3) as the actual shape data £({) and solving Eq. (3) for the entrance side strip thickness H(i). The shape prediction modeling module 111 can confirm the accuracies of the roll gap model 11 la, exit side strip thickness profile model 111b, and shape prediction model 1 lie by comparing the entrance side strip thickness H(i) found in this way and the previously measured strip thickness of the rolled material A or the strip thickness H(i) convertedfi"om h(i) as described previously. The operation of the rolling control device 110 during a rolling operation associated with the present embodiment is next described. FIG. 6 is a flowchart illustrating operations performed by the rolling control device 110 in a case where a rolling operation is performed by rotating the rolling mill forwardly. As illustrated in FIG. 6, when the rolling operation using forward rotation is started, the rolling control device 110 begins to acquire actual measurement shape data e(i) by an actual measurement of the shape using the shape meter 107 (S601). When the shape data S(i) is started to be acquired, the shape deviation calculating portion 112 computes the shape deviation between the acquired actual shape data e(i) and the target shape data that is set in the target shape setting portion 113 (S602). The target shape data outputted by the target shape setting portion 113 corresponds to the actual measurement shape data S(i) or the prediction shape data X