1. An image processing apparatus for processing an input image which uses a color material of a spot color except for process colors, comprising: a memory which stores colorimetric values of colors represented by mixing spot colors; a detector, arranged to detect a mixed color region, where spot colors are mixed, from the input image; a calculator, arranged to calculate a colorimetric value of the detected mixed color region by using density of each spot color in the detected mixed color region and the colorimetric values stored in said memory; and a color conversion unit, arranged to convert the calculated colorimetric value into color data dependent on an output device.