PURPOSE: To protect a switching element against damage by a method wherein it is judged that a drive signal is normal or not at the inverter end of a transmission line provided between an inverter circuit and a drive signal output circuit, and when it is found that the drive signal is abnormal, a laser is made to stop oscillating.

CONSTITUTION: When a drive signal is not normally given to a MOS FET 9 through the intermediary of a gate amplifier 5 due to that an optical fiber comes off from a transmitter 2 or a receiver 4 or the transmitter 2 is not turned ON because of malfunction, or when an optical signal serving as a drive signal of the MOSFET 9 becomes abnormal, the receiver 4 outputs a high or a low direct voltage output. At this point, as an input capacitor 13 of a pulse discriminating circuit 10 shuts off a direct voltage, a FET 16 is turned OFF, the output of the pulse discriminating circuit 10 becomes low, and the output of an AND circuit 11 becomes also low. In this case, a laser oscillator control circuit 1 stops the operation of an oscillation circuit 1a receiving the output of the AND circuit 11 to stop the oscillation of a laser, whereby the MOSFET 9 of the inverter circuit 6 can be protected against damage.