PURPOSE: To lessen irregularities in rotation, and to attain highly accurate rotation by obtaining the relative speed of a photosensitive drum or a sensitized paper holding drum and an exposing position, and controlling the rotational drive of them.

CONSTITUTION: A photodetector array 1 being an exposing position detecting means, and a differential amplifier 2 directly or indirectly receive modulated light with which the surface of the photosensitive drum 5 or the sensitized paper holding drum is irradiated, by a photodetector 1a, to detect the exposing position of the modulated light on the drum. A differentiation circuit 4 being a relative speed calculating means requires the relative speed of the photosensitive drum 1 or the sensitized paper holding drum and the exposing position, from electrical signals outputted by the exposing position detecting means 1 and 2, and transmits the calculated relative speed to a speed calculating circuit 5 as a rotation/drive control means. At this time, the speed calculating circuit 5 controls the rotational drive of the photosensitive drum 51 or the sensitized paper holding drum so that the relative speed obtained by the differentiation circuit 4 is prescribed speed.