To utilize an image A decoded image information which is coded irreversibly, and to efficiently code/decode the image information of an original image B by using a system capable of controlling the amount of codes that contain high-frequency components.

A differential creation section 103 produces a differential image of the image information of the image B from that of the image A. A bit-extracting section 105 expresses the absolute value of the differential information at each pixel position of the differential image by binary number and extracts from the top Mth bit to the Nth bit, and a bit-coding section 106 codes it. When the extracted bit is significant, a positive/negative coding section 108 codes positive/negative information of the differential information. A multiplexing section 109 multiplexes the encoded information as the encoded data of the image B.