PURPOSE: To reduce a packaging area by dividing fundamental frequency at frequency-dividing ratio corresponding to write density commanded from a host machine and obtaining picture element frequency required for each write density.

CONSTITUTION: When a write density selection signal S showing write density 400dpi is transmitted to a laser printer 1 from the host machine 5, a signal corresponding to the write density 40Odpi is transmitted to a frequency-dividing ratio setting circuit 7 by a selection circuit 6. The circuit 7 transmits the frequency-dividing ratio N=1/16 corresponding to the wirte density 400dpi to a programable frequency division circuit 4 based on the transmitted signal. In such a case, a signal whose fundamental frequency is 25MHz is transmitted to the circuit 4 from the fundamental frequency generation circuit 2. The frequency divider 4 divides the fundamental frequency 25MHz at the frequency- dividing ratio N to obtain the picture element frequency 1.388MHz for the write density 4O0dpi. Then, the image is formed by the picture element frequency. Therefore, the picture element frequency is obtained just by setting the frequency-dividing ratio N to the circuit 4 according to a command from the host machine 5.