1. An image compressing method for generating, from a foreground layer including N (wherein N is a natural number) kinds of foreground identifiers for identifying color information of respective pixels of a foreground corresponding to a text and/or a line art included in a color image consisting of a plurality of pixels, M (wherein M is a natural number satisfying M<N) binary images respectively corresponding to specific foreground identifiers and compressing the M binary images, comprising steps of: reducing or retaining n 1 (wherein n 1 is an integer satisfying n 1 ≧0) kinds of foreground identifiers for identifying color information of chromatic colors to m 1 (wherein m 1 is an integer satisfying 0≦m 1 ≦n 1 ); and reducing or retaining n 2 (wherein n 2 is an integer satisfying n 2 ≧0 and N=n 1 +n 2 ) kinds of foreground identifiers for identifying color information of achromatic colors to m 2 (wherein m 2 is an integer satisfying 0≦m 2 ≦n 2 and m 1 +m 2 =M).