To make high density and tone reproduction compatible by preventing tone deviation which is apt to generate at reproduction of a dark color part in a printed matter including a color image.

At an overlap printing process of four primary color (CMYK) ink, a first set using a main color part plate and a second set using a complementary color part plate are divided and further color separation data is corrected (an example of the correction expression is shown in Fig. 2) to optimize dot areas of the main color part plate and the complementary color part plate. At the overlap printing using the main color part plate and the complementary color part plate, the same (CMYK) ink is used.