A CPU (91) of an image forming apparatus includes a beam detecting part (911) that detects each of laser beams scanning by a predetermined number (e.g., four) of polygon mirrors (332) at a preset position (where a beam sensor (335) is disposed), a reference phase setting part (912) that sets a reference phase that is a phase to be a reference of the predetermined number of rotation phases of the polygon mirrors (332) based on a result of detection by the beam detecting part (911), and a phase control part (913) that controls polygon motor (330) so that predetermined number of rotation phases of the polygon mirrors (332) match the reference phase.