An image forming apparatus includes a control unit. The control unit is capable of executing a first mode for forming a toner image on the photosensitive member that rotates at a first speed and a second mode for forming a toner image on the photosensitive member that rotates at a second speed slower than the first speed, and the control unit causes the light illumination unit to perform the weak exposure in the first mode without thinning the scanning surface of the deflection scanning unit and perform the weak exposure in the second mode while thinning the scanning surface of the deflection scanning unit.