PURPOSE: To detect easily and accurately a reference signal used for regulation or the like of a reference point in the direction of the scanning line by providing a marker, which is provided in a prescribed position in the direction of the laser beam scanning near a light receiving face or on this face, and a photodetector which detects the reflected light of a laser beam on this marker.

CONSTITUTION: A photodetector 11 which receives the reflected light is attached to a marker 10 provided in a prescribed position which is a minute length apart from the light receiving face of a photosensitive drum 7 and precedes a latent image forming area AB on the photosensitive drum 7 with respect to scanning. A recessed part or a projecting part is detected by the photodetector 11 to detect the time when the laser beam scans the marker.