A decoding unit (12) decodes an image according to JPEG 2000. A simplifying unit (30) comprises an automatic transformer (32) which compares an elapsed time to a time limit at each stage of decoding the image and switches the decoding process to a simplified process if necessary. For instance, if the playback of a motion picture takes over 1/30 second, the simplified process, in which only low frequency components are decoded, is conducted by the automatic transformer (32).