An image forming apparatus includes an optical scanning device that scans a scan area on an image carrier with light flux containing image information and writes the image information onto an image area in the scan area. In the optical scanning device, a first light receiving unit receives light flux emitted from a light source and reflected by a reflecting optical unit; a second light receiving unit receives light flux that passes through an aperture of the reflecting optical unit, is deflected by a deflecting unit, and heads for outside the image area within the scan area; and a control unit controls a drive signal of the light source based on signals output from the first and second light receiving units.