To facilitate color matching under various observation environment conditions by conducting the conversion of a standard signal proper to an actual environmental condition, converting the converted standard signal into a 2nd device signal and providing an output of the converted signal.

The processing unit is made up of a color scanner 18, a color monitor 19, a color printer 21 interconnected via a color conversion section 1, an observation condition correction processing section 16, and color conversion sections 2, 3, and also made up of the color conversion sections 1 to 3, a color conversion parameter storage memory storing color conversion parameters Ps, Pm, Pp, and the observation condition correction processing section 16 converting the standard signal depending on the observing condition. Then the color conversion section 1 converts the 1st device signal into a standard signal under a prescribed observation condition, the color conversion section 2 converts the standard signal into the 2nd device signal, and the observation condition correction processing section 16 corrects the standard signal based on a prescribed function when the observation condition differs from an actual observation condition.