PURPOSE: To obtain a reverse image output with no need at all for data processing in a polygon motor driver circuit by using only one optical sensor signal for position detection according to the rotating direction of the polygon motor.

CONSTITUTION: The polygon motor is run backward by applying a signal 14 to run the polygon motor backward to the polygon motor driver circuit 11 in the case of forming the reverse image. A gate signal for selecting a beam position sensor 6-2 is applied to a sensor circuit 10 at the same instant to complete the prepn. for forming the reverse image. The signal generated when a light beam crosses the beam position sensor 6-2 is thereafter applied as a reference signal to an image signal transmitter 9. The image data for one line is successively sent from the transmitter 9 to a modulating circuit 8 in conformity with the specified clock frequency, by which the laser light is modulated according to the image signal. The image formation is executed by repeating the above-mentioned operation.