A phase shift unit provides a prescribed phase difference (pi/2, for example) between a pair of optical signals transmitted via a pair of arms constituting a data modulation unit. A low-frequency signal f₀ is superimposed on one of the optical signals. A signal of which phase is shifted by pi/2 from the low-frequency signal f₀ is superimposed on the other optical signal. A pair of the optical signals is coupled, and a part of which is converted into an electrical signal by a photodiode. 2f₀ component contained in the electrical signal is extracted. Bias voltage provided to the phase shift unit is controlled by feedback control so that the 2f₀ component becomes the minimum.