Laser driver ICs each drive different light-emitting portions and supply a driving current to one or more light-emitting portions among light-emitting portions included in a semiconductor laser. An image forming apparatus controls the laser driver ICs such that the light-emitting portions, which are each driven by a different laser driver IC, emit light beams in sequence, and measures a time interval between two BD signals generated by a BD sensor due to the light beams being incident thereon. Furthermore, based on the measured value, the image forming apparatus controls relative timings according to which the light-emitting portions emit the light beams based on image data.