Image distortion is corrected in a color printer wherein print units for a plurality of colors are aligned to form a color image by superposing images formed by the print units. A quantity of image distortion is detected, and correction data of main scan address and subscan address are calculated and stored according to the detected image distortion for each main scan address. When input color image data are corrected, printing position is corrected in combination of address change with density interpolation. When the correction data exceeds the maximum correction range, the correction data are replaced by the maximum in the correction range to utilize the capability of the distortion correction of the printer. Further, in the image data correction, after the image data are converted to data having a smaller number of gradation levels, the data are delayed according to the serial arrangement of the print units. Then, the delayed data are converted again for printing.