In an image forming apparatus, a polygon mirror, a plurality of laser generators, a first sensor, and a second sensor are disposed at a resin frame. A laser beam outputted from each of the laser generators and deflected by a deflection surface is irradiated onto a surface of a photosensitive drum to scan the surface over a scan part. The first sensor is disposed at a first position to detect a laser beam deflected by the deflection surface. The second sensor is disposed at a second position to detect a laser beam deflected by the deflection surface. A storage unit stores correspondence information indicating shift of the scan part from a reference scan part in relation to time difference between detections of the laser beam by the first and second sensor. A controller controls the laser generator to output a laser beam based on the shift of the scan part.