A color printer has means for setting toner image forming areas in the paper sheet convey direction in respect of respective colors, means for detecting a reference position on a photosensitive belt, and image formation start position setting means responsive to the reference position to set a toner image (electrostatic latent image) formation start position in the rotation direction of the photosensitive belt. When the paper sheet length is longer than the outer peripheral length of an intermediate transfer member, the image formation start position setting means sets, for colors other than the final color, a printing area which is shorter than the outer peripheral length of the intermediate transfer member and sets, for the final color, a printing area which begins immediately after a seam position of the photosensitive belt and conforms to the paper sheet length. Through the above setting, when toner images of two colors are superposed to print a color image, the printing area of the final color can be set to a desired range in the paper sheet length direction regardless of the outer peripheral length of the intermediate transfer member, thereby improving the printing speed and preventing degradation of quality.