A laser printer comprises two reference clock generators to define the rotational speed of a polygon mirror (main scanning rotational mirror); namely, a first reference clock generator which generates a clock signal whose frequency is set so as to correspond to a scanning line density for a millimeter system, and a second reference clock generator for generating a clock signal whose frequency is set so as to correspond to a scanning line density for an inch system. One of these clock generators is selected in accordance with the scanning line density of received image data. As the rotational speed of the polygon mirror decreases, a decrease and an increase in the image data density occur in the directions of main- and sub-scan, respectively, whereas as the rotational speed of the polygon mirror increases, an increase and a decrease in the image data density occur in the directions of main- and sub-scan, respectively.