A monochrome image forming apparatus includes an image data supply unit for the color image forming apparatus that has a function to write input color image data including plural primary color data components into color buffers; and a color converter for a color image forming apparatus in which a color conversion unit reads the input color image data from the color buffers, performs an UCR process and a black generation process. The image data supply unit writes input monochrome image data of a single color to a monochrome buffer. In an eco printing mode, the color converter reads out the input monochrome image data to the color conversion unit as the plural primary color data components of the color image data, and performs the UCR process and the black generation process so as to make a toner consumption amount smaller than that in a normal printing mode.