PURPOSE: To enable a high quality image to be formed at a high speed by a low costed simple circuit structure.

CONSTITUTION: A first timing signal generating means 1 and a second timing signal generating means 2 delay a write timing signal by a plurality of time units. An image data control means 5 selects a first timing signal and a second timing signal respectively from the first timing signal generating means 1 and the second timing signal generating means 2 according to gradation of an image data. A pulse width modulation signal generating means 3 outputs a pulse signal of a pulse width which rises with the first timing signal and falls with the second timing signal to an optical control means 4. The optical control means 4 generates an optical beam modulated with the pulse signal.