1. An image coding method for coding, on a per-block basis, pictures each including slices, the method comprising: estimating a current motion vector which is a motion vector of a current block to be coded and specifies a reference block included in a reference picture; generating a 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 coded which includes the current block; subtracting the prediction image block from the current block to generate a difference image block; and coding the current motion vector and the difference image block.