PURPOSE: To obtain an operation point control method at the time of driving an optical modulator using a lithium niobate substrate in an optical communication system.

CONSTITUTION: By this circuit, bias voltage applied to the optical modulator 2 is controlled by detecting mean output power of an optical modulation signal from the optical modulator 2 at the time of driving the optical modulator using the lithium niobate substrate and comparing it with reference voltage. Since the mean value power of an optical signal modulated by optical modulation is increased according to increase in a drift amt. in the optical modulator 2, an operation point is monitored from the mean power of the optical signal.