In an image processing apparatus, a pixel data processing section quantizes pieces of pixel data that represent individual pixels, and a pixel data storage section temporarily stores the quantized pixel data. A data monitoring section refers to the pixel data for a plurality of pixels stored in the pixel data storage section, and specifies a bit plane having the greatest bit plane number of bit planes that include at least one "1". An effective data select section feeds only pieces of bit data for bit planes whose bit plane numbers are equal to or smaller than the specified bit plane number of the respective pixels among the data in the pixel data storage section, to a bit plane data processing section. The bit plane data processing section codes only the data fed from the effective data select section.