PURPOSE: To prevent the deterioration of an image caused by the fluctuations of an operation speed without using a complicated and inexpensive system like a conventional servo mechanism.

CONSTITUTION: In order to determine the speed of a light receiver surface, a shaft encoder 32 attached to the vicinity of the light receiver surface is used as a sensor. The fluctuations of the speed of the light receiver surface are determined from the relation with a reference speed by a comparator 76 connected to the sensor 32 electrically and a signal showing the quality coefficient of an image is formed corresponding to the fluctuations of the speed by the converter connected to the comparator 76 and the modulation of the beam of a scanning means is controlled in response to the quality coefficient of the image by a modulation altering circuit 82 so as to correspond to the speed of the light receiver surface.