An image processing apparatus includes an emitter which has a plurality of light emitting devices arranged in a sub-scan direction, and a former which forms an image on a photosensitive member by multi-passed scanning an identical region on the photosensitive member in a main scan direction using light beams output from the respective light emitting devices of the emitter. An input section inputs image data of an image to be formed by the image processing apparatus. A divider divides the input image data into image data for respective scans in the multi-passed scans using a mask pattern generated based on amounts of deviations from scan lines of the light beams output by the respective light emitting devices of the emitter. A supplier supplies the divided image data to the emitter in accordance with scans of the former.