PURPOSE: To protect a semiconductor laser from breakdown when excessive driving current is applied, by detecting and breaking the driving current of the semiconductor laser momentarily.

CONSTITUTION: Driving current is converted to voltage by an amplifier 4 then the value thereof is compared with a reference voltage Vref2 by a comparator 7. When excessive driving current is supplied to LD1, the driving current to LD1 is broken at once and LD1 is thereby prevented from breakdown due to excessive current. Since the reference voltage Vref2 is controlled by a temperature detecting element 8 and a voltage controlling circuit 9 according to ambient temperature change, light output is maintained constant by the threshold value current change of LD1 due to the temperature change. Therefore, if driving current increases, it can be prevented from breaking by error. If the excessive driving current is partial, driving current can be supplied again to LD1 after a specified time period.