A duplicated part of identical images in an encoded image data master is judged to be selected. Image data to be reused is generated by adding a storage instruction to encoded image data of the duplicated part of the first identical image, and converting encoded image data of the duplicated part of the second identical image into a reuse instruction. In a reproduction process, decoded data in the encoded image data part including the storage instruction is stored in a vacant area and the decoded data is read at the point of the reuse instruction.