In encoding/decoding of moving images, the efficiency of encoding is enhanced and the expansion is facilitated, while the increases in the capacities of frame memories and in bands are suppressed. A bit length expanding converter (102) converts a target image, the pixel values of which are expressed by use of a bit length N, to an image the pixel values of which are expressed by use of a bit length M. A compressor (110) encodes the image as converted. A decompressor (111) decompresses the image as encoded. A bit length contracting converter (113) converts the image as decompressed to a reproduced image the pixel values of which are expressed by use of a bit length L shorter than the bit length M. The reproduced image is then stored, as a reference image, into a frame memory (114).
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