An image forming apparatus includes an optical scanning unit including a rotatable polygonal mirror and a detecting portion; and a controller. A rotation center of the rotatable polygonal mirror is made eccentric with respect to a center of gravity of the rotatable polygonal mirror by 7.5 μm or more. The controller identifies one of reflecting surfaces of the rotatable polygonal mirror as a reference surface by using a signal outputted from the detecting portion for each of the reflecting surfaces of the rotatable polygonal mirror and a period of signals from light beams reflected by consecutive two reflecting surfaces with respect to a rotational direction of the rotatable polygonal mirror.