PURPOSE: To prevent image quality from being deteriorated owing to addition of a pattern, while a printer may be interrupted by addition of a specific pattern.

CONSTITUTION: By control from a CPU14, a marking process part 53 adds concentration by adding density by a specific density partially only to Y color data in M, C, Y, Bk colors, which is incorporated into an ID recognition marking for recognizing a device as a preliminarily determined specific pattern. Then, conversion to an optimum density to which a γ-characteristic is added is carried out with a γ-correcting part 21, and an image data after conversion is printed.