An image forming apparatus capable of carrying out color misregistration correction at an appropriate time and providing a high-quality image free from color misregistration. A toner image is formed on the basis of an input image information signal. A width of the toner image is determined on the basis of the input image information signal, before forming the toner image in a predetermined area. The width of the toner image formed in the predetermined area is detected. The detected width of the toner image is compared with the determined width of the toner image. It is judged on the basis of the comparison result whether or not color misregistration is present.