An image forming apparatus, including: a polygon mirror; a first optical sensor; a second optical sensor; and a controller configured to (a) determine a second-beam exposure standby time which is a time period from a timing when the first beam deflected by a first surface of the polygon mirror has been detected by the first sensor to a timing when scanning exposure of the second beam deflected by a second surface of the polygon mirror is started, based on a measured time which is a time period from the timing of detection by the first sensor to a timing of detection of the second beam by the second sensor and (b) start the scanning exposure of the second beam deflected by the second surface when the second-beam exposure standby time elapses after the first beam deflected by the first surface has been detected by the first optical sensor.