1. An image decoding method for decoding, on a per-block basis, pictures each including slices, the method comprising: decoding a current motion vector and a difference image block, the current motion vector being a motion vector of a current block to be decoded and specifying a reference block included in a reference picture, and the difference image block indicating a difference between the current block and a prediction image block; generating the prediction image block by allocating a value of an inside pixel to an outside pixel, the inside pixel being a pixel located inside an associated slice, the outside pixel being a pixel located outside the associated slice and included in the reference block specified by the current motion vector, and the associated slice being a slice included in the reference picture and corresponding to a current slice to be decoded which includes the current block; and adding up the difference image block and the prediction image block to reconstruct the current block.