When N kinds of foreground identifiers for identifying color information of a foreground of a color image is to be reduced to M smaller than N, foreground identifiers corresponding to similar chromatic colors are merged and foreground identifiers corresponding to similar achromatic colors are merged but a foreground identifier corresponding to a chromatic color and a foreground identifier corresponding to an achromatic color are not merged. Thus, image quality degradation derived from color change from an achromatic color to a chromatic color or vice versa may be suppressed. On the basis of a foreground layer including the M kinds of foreground identifiers, M binary images respectively corresponding to the M kinds of foreground identifiers are generated, each of the binary images is subjected to lossless compression and a background layer is subjected to lossy compression.