PURPOSE: To enable the uniform exposure of a photoreceptor irrespectively of the uneven contamination of an optical system, by providing an adjusting means which changes the aperture width of a slit according to the detection value of a photodetector which detects the quantity of the light reflected from a standard white plate provided in a non-image part.

CONSTITUTION: When contamination progresses on account of an increase in the amt. of the toner stuck on both end parts of an optical system 4 in the transverse direction thereof, the decrease in the quantity of light in the system 4 is detected with a photodetector 7 and a control part is operated by the detection signal of the detector 7 so as to increase the supply current to a light source lamp 3; at the same time, a movable piece 5d is oscillated at the alternate long and short dash lige as shown by a driving part 5e, and the slit width W at both ends of an adjusting slit 5 is increased. As a result, the decrease in the exposure on a photoreceptor 6 owing to the average contamination of the toner on the system 14 is compensated and the uneven distribution of the exposure on the photoreceptor 6 owing to the contamination of the toner in both end parts in the transverse direction in the system 4 is corrected and the copy having homogeneous image quality is obtained.