Color converters are often implemented as hardware due to being required to perform high-speed processing; however, the scale of hardware of a color converter that corresponds to five or more colors becomes large and its cost rises. Since acquisition of a color converter that corresponds to four colors is easy, it is desirable to provide for the handling of five or more colors effectively using a four-color color converter. Thereby, when converting RGB image data into CMYKLcLm image data, a color converter, which almost simultaneously color-converts RGB into CMYK, executes RGB to LcLm color conversion and then executes RGB to CMYK color conversion. Then, the color conversion result is held in memory and CMYKLcLm image data is generated.