From color image data read by an image pickup device (116), concentration histograms are generated for each of RGB color components, and with the specific color concentration histogram used as reference, other color concentration histograms are shifted in the concentration axial direction, respectively; then, other color concentration histograms are stretched in the concentration axial direction with the minimum concentration value used as reference, superimposed areas of concentration histograms by combinations of two color components after stretch-processing are calculated and derived, respectively, and based on the shift rate and stretch ratio which maximize the total of superimposed areas obtained respectively, each color component of each pixel of the color image data is converted.