To form an image without distortion regardless of the scanning irregularity of scanning light, by generating an image forming clock with the timing of the detected scanning light and modulating a laser beam for exposure.

One of two scanning light beams generated in a laser output part 40 is introduced to an optical fiber group 73 in a scanning light introducing part 71. A clock generating circuit 33 generates an image forming clock independent of the timing of the scanning light. A clock control circuit 32 issues a changeover control signal except an effective area and the clock from the clock generating circuit 33 is supplied to an image forming processing circuit 31. In the effective area, the clock control circuit 32 outputs a changeover control signal and supplys a clock complied with scanning light from a waveform shaping circuit 34 to the image forming processing circuit 31. In this case, since a detecting signal is obtained on a position scanned by the scanning light, a clock pulse is obtained with the timing of a pixel to be actually formed.