A pattern data storage stores a plurality of different pattern data having the same data size as bit planes. An image compressing/decoding unit calculates code amounts of the respective encoded bit planes and successively adds the code amounts of the respective bit planes in decreasing order from that of the bit plane of the most significant bit to such an extent that the sum does not exceed a limited code amount. Then, the image compressing/decoding unit selects the bit planes with highest degrees of coincidence by comparing the bit planes other those whose code amounts were added to such an extent that the sum does not exceed the limited code amount and the respective pattern data stored in the pattern data storage and replaces the bit planes by the selected pattern data.