An image forming apparatus includes a detector for detecting one of position and density information of a detection image based on a signal corresponding to a difference between a value of a detection signal corresponding to a first position of one of the detection image on an image carrier and a surface of the image carrier and an average value of a value of the detection signal corresponding to a second position on a downstream side with respect to the first position in a moving direction of the detection image and a value of the detection signal corresponding to a third position on an upstream side with respect to the first position in the moving direction during a time when the detection image formed on the image carrier passes through an irradiation region of an irradiation device.