PURPOSE: To prevent excessive voltage being applied to a semiconductor laser by controlling and monitoring a semiconductor laser based on a feedback amount of a photodiode and by stopping current supply to the semiconductor laser when the feedback amount is less than a standard value.

CONSTITUTION: Should a circuit and LD1 are disconnected, the output current of PD2 stops and the output voltage of an amplifier 3 lowers. If a comparator 7 detects the above condition, the output of a flip-flop 8 changes from High level to Low level, a gate circuit 9 closes, a transistor 5 turns ON through a buffer gate 4, and a transistor 6 turns OFF thereby. Thus the current supply to LD1 can be stopped. Moreover, a switch 18 turns ON by the output of the gate circuit 9 thus grounding a collector of the transistor 6 and preventing excessive voltage over the rating from being applied to LD1.