PURPOSE: To change easily the scanning line density by decreasing the frequency of generation of a scanning start detecting signal less than the frequency of scanning of a rotary polygon mirror.

CONSTITUTION: A counter 13 counts a light detecting signal 107 generated at each time point of start of each scanning from a rotary polygon mirror, reads a synchronizing signal 111 and applies the signal to a control circuit 11. The count value of the counter 13 is changed by a scanning line density control circuit 14 and the signal whose period is changed is transmitted from the counter 13. An output of a blanking signal generating circuit 9 and a picture buffer memory 12 is applied to an optical modulator as a picture signal via an OR circuit 10.