PURPOSE: To generate a proper synchronizing signal by adjusting an output of a laser diode for synchronization.

CONSTITUTION: An output of a synchronization laser diode 14 is monitored by a photodiode PD, a current flowing to a photodiode PD is changed into a voltage by a variable resistive element R1 and the voltage is fed to a differential amplifier 14, and its difference signal is used to apply feedback control to the synchronization laser diode 14. When a synchronizing signal is in error, a switch SW is closed, a current flowing to the photodiode PD is changed into a voltage by variable resistive elements R1, R2 connected in parallel and the voltage is fed to the differential amplifier 44. In this case, the voltage is decreased by the variable resistive element R2 and then, an output of the synchronization laser diode 14 subject to feedback control is increased.