An image forming apparatus includes: a speed setting portion configured to set a rotation speed of a drive motor to one of a plurality of predetermined specific speeds; a measurement processing portion configured to, in each of a plurality of detection cycles in which light is detected after scanning by a polygon mirror, measure an interval between a detection timing of the light and an output timing at which a drive signal input to the drive motor of the polygon mirror first after the detection timing, is output; and an identification processing portion configured to identify a reference reflection surface that corresponds to a reference interval, based on the measured interval and the reference interval corresponding to a specific speed at a time when the measured interval was measured, the reference interval being among reference intervals that have been set in advance for each of the specific speeds.