1. A method for controlling a thermal transfer printer, comprising the steps of: dividing color image data to be printed into image data of two sub-images containing an overlapping region and having edges that coincide for each of a plurality of color inks transferred to paper; converting color values of the color image data in the overlapping region by using a color conversion factor group created in advance for a plurality of different positions on the overlapping region so as to cancel out a color change that occurs in the overlapping region when the two sub-images are transferred with one overlapping the other; correcting the image data of the two sub-images by adjusting converted color values in the overlapping region by using a correction factor for print density at each position on the overlapping region; and forming a color image to be printed by sequentially transferring the two sub-images in accordance with corrected image data of the two sub-images so that the two sub-images overlap at the overlapping region.