PURPOSE: To detect a failure in power supply voltage in an early stage and protect a laser diode by securely shutting current flowing to the laser diode.

CONSTITUTION: A three-terminal regulator 14 converts a power source DC 24V input via a resistor R1 into a power source 12V for LD. A voltage monitor 15 monitors voltage at its input end (B point) and outputs a failure signal when voltage failure is detected. A current breaker 16 shuts the power source 12V according to the failure signal or a shut signal. A first invention refers to difference in upper and lower thresholds due to hysteresis of the voltage monitor 15 which is larger than drop voltage due to the resistor R1, while a second invention refers to a CPU 1 not shown in Fig. 1 which senses the failure signal and outputs a shut signal of predetermined timing, wherein the power source 12V can be securely shut by each of the inventions.