An image decoding device including an intra prediction unit for, when a coding mode for a coding block is an intra coding mode, carrying out an intra-frame prediction process on each of processing blocks to generate a prediction image, each of the processing blocks being the coding block or a sub-block obtained by dividing the coding block, wherein, when an intra prediction parameter indicates a vertical prediction process and a size of the processing block is less than a predetermined size, the intra prediction unit performs a specific processing and when the intra prediction parameter indicates the vertical prediction process and the size of the processing block is greater than or equal to the predetermined size, the intra prediction unit performs a specific processing.