PURPOSE: To speed up image formation in simple construction by conducting the logic operation of a clock signal and its delay signal and forming a binary modulation signal.

CONSTITUTION: An oscillation circuit 91 of a modulation circuit generates an original clock having sufficiently higher speed than the clock of an image signal, and a frequency division circuit 92 divides the frequency of a flip-flop circuit and outputs it as a clock signal. Since this signal is reset when a beam detection circuit outputs a BD signal, it is synchronized with the BD signal. A switch circuit selects one of the clock signals to be inputted from a delay circuit 93 in accordance with an image signal V to be inputted, and outputs the selected one. the logic calculation of the delayed signal and the original clock signal is made to form a binary modulation signal. Accordingly, a driving circuit turns on/off the semiconductor laser based on the modulation signal and the exposure of a photosensitive drum is made with a pulse-modulated beam. Thus, the image can be formed at high speed in the simple construction.