Abstract: In optical scanning system of laser printer, laser lights are scanned at uniform speed on a photosensitive plane through the projection lens which brings focuses on the photosensitive element. The f.multidot..theta. correction technique is required so that the light reflected from the rotating mirror at the angle .theta. formed between the light ray and the light axis may be made incident at a position c constant times as large as the angle .theta. onto the photosensitive drum. The f.multidot..theta. correction is, without constituting and using the known optical system by a plurality of lenses, electrically performed by attaching to the rotating polygon mirror a chopper grooved at intervals, corresponding to the uniform speed correction to generate the pulse signal, thereby obtaining the optimum width of picture line.