PURPOSE: To prevent the deterioration of picture quality caused by the fluctuation of the irradiated light quantity due to the axial direction position of the photosensitive body surface by correcting the image data based on the irradiation light quantity measured in each of parts divided from an axial direction scan range of the photosensitive body surface.

CONSTITUTION: When a copying operation is started, the light quantity detectors 41-43 are set at both end parts and the center part of an axial direction scan range of a photosensitive drum with the movement of each detector holder. The laser light is irradiated by a laser scan unit 23, and the detectors 41-43 measure the light quantity at each measuring position. The measured light quantity is stored in a storage part 51. An image processing part 54 performs the A/D conversion of the output of a CCD line sensor 18. Then the part 54 carries out the shading correction processing of the A/D converted multilevel picture element data together with the correction processing of the fluctuation of the laser irradiation light quantity due to the axial direction position of the photosensitive drum surface, the color correction processing, the color conversion processing, the density processing, the multilevel processing, etc. The picture element data outputted from the part 54 are sent to the unit 23 and converted into the optical signals. These signals are outputted.