It is difficult to adjust a black plate while maintaining accurate color reproduction in a color printer that forms an image using C, M, Y, and K color agents.Upon generating a conversion condition used to convert a signal on a uniform color space into a CMYK signal which depends on a printer, the signal on the uniform color space according to a colorimetric value is temporarily converted into an RGB signal, and the RGB signal is then converted into a CMYK signal. After that, a black amount adjustment unit (2104) adjusts a K value using a black amount adjustment function based on a user's instruction.