Electric charge remaining on a photoreceptor after transfer causes image deterioration. According to one conventional method widely used in view of this problem, erase light is emitted onto a photoreceptor after the transfer to remove the electric charge remaining on the photoreceptor. However, such a method poses the following problem: emission of erase light leads to light-induced fatigue of the photoreceptor, and as a result, the photoreceptor which has deteriorated from the light-induced fatigue wears down due to abrasion and the like with a cleaner blade, resulting in a shorter lifetime of the photoreceptor. Thus, the thickness of the photosensitive layer is detected to acquire the amount of thickness decrease, and the amount of the erase light is reduced according to the acquired amount of thickness decrease.