Adopted is a decoder, in which on condition that a prediction block shown by vector information extracted from a data stream, and a decode-target block have an overlap where respective pixels overlay each other, pixel information of an already-decoded portion at a distance of an integer multiple of a vector provided by the vector information from the overlap is made a prediction signal instead of the overlap, and the prediction signal is added to difference image data taken from the data stream to generate reproduction image data. The decoder is adopted for an intra-frame decoder, a local decoder of an encoder, and the like. According to a fundamental rule concerning a repetitive pattern of an image, a pixel at a distance of an integer multiple is a like pixel, and therefore the process of decoding can be performed efficiently.