To make the operation of an ASIC surer when the ASIC is actuated by performing control without stopping the ASIC even if the actuation of the ASIC is delayed.

As the power source is turned on, the ASIC 52 and a CPU 51 are actuated and the CPU supplies a laser-on signal to a laser driving circuit instead of the ASIC. Consequently, a semiconductor laser element emits a laser beam. The laser beam is deflected by a deflecting device for scanning and then detected by a beam position detection sensor. A horizontal synchronous detection circuit generates a horizontal synchronizing signal with the detection signal from the sensor. The ASIC begins to send image data for control out to the laser driving circuit in response to the horizontal synchronizing signal. When the image data for control begin to be sent out from the ASIC, the ASIC responds to the horizontal synchronizing signal which is generated, line by line, to perform beam control based upon the image data.