1. An image forming apparatus comprising: an image forming unit adapted to form images of a plurality of colors in a manner in which a predetermined pattern of a predetermined color is registered thereon and superimpose those images to form a color image; a controller configured to control said image forming unit so as to form a patch image of each color for density detection; and a detector configured to detect the density of each of said patch images; wherein said controller controls said image forming unit so as to superimpose said predetermined pattern of said predetermined color upon the patch image of a predetermined color and not to superimpose said predetermined pattern of said predetermined color upon the patch images of the other colors.