An image forming apparatus including a polygon mirror that deflects a light beam includes: a light emitter that emits the light beam; an optical sensor that is disposed at a position on which the light beam deflected by the polygon mirror is incident; a detector that detects a minimum level being a light intensity level of the light beam, the lowest within a range detected by the optical sensor; a switcher that switches a light intensity level of the light beam emitted from the light emitter until the detector detects the minimum level; and a determiner that determines a degree of contamination of a component on an optical path of the light beam extending from the light emitter to the optical sensor, on the basis of the minimum level detected when the light intensity level is switched by the switcher.