A monochromatic image signal is made from a color image signal. Values of brightness of pixels in a color border region of a type where two color regions which are small in brightness difference and large in chromaticity difference are positioned side by side are changed to new values different from those represented by the image signal component for the pixel of the color image signal, and a monochromatic image signal is generated with values of brightness of pixels in the color border region set to the new values and values of brightness of pixels in regions other than the color border region set to those as represented by the image signal components for the pixels of the color image signal.