PURPOSE: To improve the expressing capability of colors of a low chroma while holding the expressing capability of high-density gray components due to an Indian ink by attaining an Indian ink signal where gray components and chromatic components are taken into consideration.

CONSTITUTION: A maximum value of color signals C0, M0, and Y0 is detected by a minimum value detecting circuit 2, and a maximum value of them is detected by a maximum value detecting circuit 7. A threshold operating circuit 8 outputs a threshold on a basis of minimum and maximum value detection values. A gray component separating circuit 9 operates and outputs gray components δ which should be expressed with a BK ink, on a basis of the threshold and the minimum value detection value. Gray components δ are multiplied by a grain β in a multiplier 6 to generate a BK ink signal. Thus, the expressing capability of colors of a low chroma is improved while holding the expressing capability of high-density gray components due to the Indian ink.