Tristimulus values of each of pixels in an image, which is represented by an image signal, are transformed into chromaticity values, and mean values of the chromaticity values having been calculated with respect to the pixels in the image are calculated. The mean values of the chromaticity values are transformed into physiological primary colors. The thus obtained physiological primary colors are regarded as the white of a photographing illuminant, under which a photographing operation was carried out. The chromaticity values are transformed such that the thus obtained physiological primary colors may coincide with the physiological primary colors of a reference photographing illuminant. The chromaticity values, which have been obtained from the transformation, are then transformed into RGB color signals. The thus obtained RGB color signals are subjected to image processing and then used for reproducing a visible image.