An image forming apparatus includes a control unit: starting up a rotary polygon mirror based on a first signal output by a driving unit thereof; causing a light source to emit a laser-beam while controlling a rotational speed of the rotary polygon mirror based on the first signal to acquire a second signal output from a detecting unit of the laser-beam; turning off the light source after acquiring a phase relation between the first and second signals; causing a charging unit to charge a photosensitive member; making the laser-beam to enter onto the detecting unit without exposing the photosensitive member based on the phase relation in response to the rotational speed reaching a target speed; controlling the rotational speed based on the second signal in response to the laser-beam being entered; and starting image formation in response to the rotational speed converging within a predetermined range including the target speed.