An image forming apparatus for forming a tone image using a density pattern method has a ROM for storing a density pattern corresponding to each of a number of tones and an ideal density value of each density pattern, and a sensor for detecting the optical density of an image that has been formed by an image forming unit that forms the image by an electrophotographic method. An image is formed by the image forming unit based upon the density pattern corresponding to each tone, and the density pattern corresponding to each tone is modified based upon optical density, which has been obtained by detecting the density of the image by the sensor, and the ideal density value of each density pattern.