An object is to accurately determine a factor that drops the quantity of laser light. An image forming apparatus is provided, which has: a laser diode that includes a front side emission point to which drive current is supplied and from which rear light is emitted; a photosensitive drum to which the rear light is emitted; a detection portion that detects a value on a surface potential of the photosensitive drum; and a determination portion that determines an abnormality of the laser diode, and the detection portion detects, a plurality of times, a value on the surface potential of a portion of the photosensitive drum to which the rear light is emitted, and the determination portion determines whether the laser diode is abnormal based on a change amount of the value on the surface potential detected by the detection portion.