PURPOSE: To simplify handling by obtaining the detection signal from the laser beam modulated with oscillation signal by using a photoelectric transducer, when the phase of the detection signal is opposite to that of the reference oscillation signal, the polarity of the modulation signal to be supplied to the electro- optical light modulator is converted.

CONSTITUTION: The output light L1 modulated by an electro-optical light modulator 2 using the oscillation signal is divided by a beam splitter 3, and the transmitted light is led to a light modulator 4. The reflected light is led to a photoelectric transducer 10, and is converted to an electrical signal so that a signal of the oscillation frequency is detected. The detected signal is supplied to a synchronization detecting part 20 via an amplifier 11, and its synchronization with the reference is detected by using the oscillation signal that is supplied through an amplifier 22. If the phase is opposite, the polarity changeover ion control signal from the synchronization detecting part 20 is supplied to the changeover switch 15 of a controlling part 16. By this signal, the polarity of the modulation signal being impressed to the modulator 2 is reversed. Thus, the stability control of the output light L1 of the modulator 2 is made possible.