Data for color conversion which is previously stored in a ROM is transferred to a three-dimensional look-up table when a color copier is turned on. By referring to R, G, B, signals, which have been previously stored in a buffer memory, in accordance with M, C, Y, Bk signals output in synchronization with the printing operation of a printer, a CPU rewrites the data in the look-up table to appropriate data (M, C, Y, Bk output signal values). A color signal processing circuit refers to the updated look-up table and outputs M, C. Y, Bk color signals that correspond to entered R, G, B signals.