1. An image encoding device comprising:
an intra predictor for, when a coding mode corresponding to one of coding blocks into which an inputted image is divided is an intra coding mode, carrying out an intra-frame prediction process on each block which is a unit for prediction of the coding block to generate a prediction image; and
an encoder for encoding coding mode information and an intra prediction parameter indicating an average prediction, wherein
when the intra predictor carries out the average prediction, the intra predictor carries out a filtering process on target pixels of intra prediction located at an upper end and a left end of the block, the filtering process using an intermediate prediction value, which is an average value of adjacent pixels of the block, and at least one adjacent pixel of the target pixel, and wherein
the intra predictor sets a filter coefficient to 1/2, associated with the intermediate prediction value for a target pixel at an upper left corner of the block, and sets a filter coefficient to 1/4, associated with an adjacent pixel adjacent to an upper side or a left side of the target pixel.
2. An image decoding device comprising:
a decoder for decoding coding mode information and an intra prediction parameter; and
an intra predictor for, when the coding mode information associated with a coding block is an intra coding mode, carrying out an intra-frame prediction process on each block which is a unit for prediction of the coding block to generate a prediction image, wherein

when the intra prediction parameter indicates an average prediction, the intra predictor carries out a filtering process on target pixels of intra prediction located at an upper end and a left end of the block based on an intermediate prediction value, which is an average value of adjacent pixels of the block, and at least one adjacent pixel of the target pixel, and wherein
the intra predictor sets a filter coefficient to 1/2, associated with the intermediate prediction value for a target pixel at an upper left corner of the block, and sets a filter coefficient to 1/4, associated with an adjacent pixel adjacent to an upper side or a left side of the target pixel.
3. An image encoding method comprising:
carrying out an intra-frame prediction process on each block which is a unit for prediction of a coding block to generate a prediction image, when a coding mode corresponding to the coding block into which an inputted image is divided is an intra coding mode; and
encoding coding mode information and an intra prediction parameter indicating an average prediction, wherein
when the average prediction is carried out, a filtering process is carried out on target pixels of intra prediction located at an upper end and a left end of the block which is a unit for prediction of the coding block, the filtering process using an intermediate prediction value, which is an average value of adjacent pixels of the block, and at least one adjacent pixel of the target pixel, and wherein
the filtering process sets a filter coefficient to 1/2, associated with the intermediate prediction value for a target pixel at an upper left corner of the block, and sets a filter coefficient

to 1/4, associated with an adjacent pixel adjacent to an upper side or a left side of the target pixel.
4. An image decoding method comprising:
decoding coding mode information and an intra prediction parameter; and
carrying out an intra-frame prediction process on each block which is a unit for prediction of a coding block to generate a prediction image, when the coding mode information associated with the coding block is an intra coding mode, wherein
when the intra prediction parameter indicates an average prediction, a filtering process is carried out on target pixels of intra prediction located at an upper end and a left end of the block which is a unit for prediction of the coding block, the filtering process using an intermediate prediction value, which is an average value of adjacent pixels of the block, and at least one adjacent pixel of the target pixel, and wherein
the filtering process sets a filter coefficient to 1/2, associated with the intermediate prediction value for a target pixel at an upper left corner of the block, and sets a filter coefficient to 1/4, associated with an adjacent pixel adjacent to an upper side or a left side of the target pixel.