To reduce time or power required for various kinds of processing, such as encoding, of the unit of a bit plane by processing only required data in the relevant processing.

Image data representing individual pixels are temporarily stored in a pixel data storage part 201 after quantization in a pixel data processing part 200. While referring to pixel data for a plurality of pixels stored in the pixel data storage part 201, a data monitoring part 202 specifies a bit plane of a maximum bit plane number out of bit planes including at least one "1". In the data within the pixel data storage part 201, a valid data selecting part 203 delivers to a bit plane data processing part 204 only the bit data of ≤ bit plane number specified in the above step in the individual pixels. The bit plane data processing part 204 applies encoding processing only to the delivered data.