To attain the compression by which decoding with a higher compression rate, at a higher speed, at a lower cost and with high quality is attained in the case of compressing multi-value image data.

A difference value calculation section 200 assigns a code in response to a frequency of incidence to a difference value between each picture element of multi-value image data and its preceding picture element so as to code the data and the result is stored in a page memory 100. Then a density value calculation section 400 decodes sequentially code data in the page memory 100 and integrates obtained differences to decode the multi-value image data.