The average value of each of yellow, magenta and cyan color image data of a color-to-be-converted which data are obtained at the time of pre-scanning of a copying machine and stored in memories, respectively, is calculated (S5), and then, predetermined set values are respectively added to and subtracted from the calculated average values of yellow, magenta and cyan (#5, #20) to obtain upper and lower limit values of the conversion range. When the lower limit value obtained by subtraction is below a predetermined lower limit restriction value, the lower limit restriction value is set as the lower limit value of the conversion range (#10, #30). With this feature, when an extremely pale color is specified as a color-to-be-converted in a region other than a background portion of a color original where no letters, etc. are printed, the color of the background portion is not converted together with the specified color.