SCANNING OPTICAL SYSTEM EVALUATING DEVICE

PROBLEM TO BE SOLVED: To precisely evaluate a scanning optical system over the whole area in main scanning direction. SOLUTION: An area sensor 9 is movably provided in the scanning direction of a laser beam scanned by a polygon mirror 4. The detection signal of the laser beam by this area sensor 9 is read to a buffer memory 13 by a CCD driving circuit 12. Further, the main scanning directional signal inputted to the buffer memory 13 is copied in sub-scanning direction by an image composing part 14, and extended to a two-dimensional image. A comparative arithmetic part 16 determines the differential value between the two-dimensional image and a master image. A host computer 10 detects the abnormality of the laser beam by the differential value and evaluates a scanning device.