An image forming apparatus includes: a light source; a light-source driving unit; a photosensitive member including an image forming region; a determining unit determining a level of a rotation speed of the photosensitive member; a scanner unit exposing the photosensitive member by deflecting the light beam from the light source by a deflecting unit; a sensor receiving the light beam and outputs a detection signal; and a control unit controlling the image forming apparatus based on the detection signal output from the sensor. When the determining unit determines that the rotation speed is low, the light-source driving unit is configured to drive the light source to emit the light beam when a deflecting direction of the light beam is oriented toward the sensor and not to drive the light source to emit the light beam when the deflecting direction of the light beam is oriented toward the image forming region.