Light having components of frequencies f0, f+1 and f−1 outputted from an optical modulator (10) is monitored, a second light detection means (14b) measures the power P2 of all the components, and a first light detection means (14a) measures the power P1 with frequency f0 component cut out by a filter means (13). Based on these light receiving powers (P1) and (P2), phase differences imparted by the respective DC electrodes of Mach-Zehnder optical waveguides (MZ-A, MZ-B, MZ-C) of the optical modulator (10) are controlled. The control is performed to minimize the light receiving power (P1) and to maximize the light receiving power (P2).