PURPOSE: To define a color part at one part as a mask area among the plural colors of comparatively close hue and to exclude the other color part from the mask area by preparing a mask corresponding to the value of a first coefficient provided concerning each base of respective picture elements in a color image.

CONSTITUTION: The image data of a color original are read by a reading scanner 36 and stored in an original image memory 22. The image stored in the original image memory 22 is displayed on a color CRT 38 and on that color image, a user respectively designates (n) pieces of object colors to be clipped and (m) pieces of colors out of the object. A matrix arithmetic part 50 respectively calculates color vectors for the object colors and the colors out of the object and performs processing to calculate a matrix. A multigradation mask arithmetic part 52 reads the picture element values of respective picture elements in the color image from the original image memory 22, calculates the coefficients corresponding to the bases, extracts the coefficients of the first color vectors from (n) pairs of provided coefficients, calculates the maximum values of those coefficients and calculates the minimum value among (m) pieces of maximum values.