After mixing the signal light and the local oscillation light with a mixing circuit, two intersecting polarization components are divided by a polarization splitter and respective components are subjected to heterodyne detection by optical receivers. Thereafter, in an electric signal stage, the intermediate frequency signal outputted from the optical receivers are applied to a 90-degree hybrid coupler and the output signal from one of the output ports is demodulated by a demodulator. On the other hand, an intermediate frequency signal outputted from the optical receivers are subjected to a subtraction to provide a principal axis angle monitor signal for the signal light. The signal outputted from the output port from the 90-degree hybrid coupler is used to monitor the elliptic ratio of the signal light and these monitor signals are applied to the polarization control circuit. The polarization control circuit drives the polarization operation apparatus based on said monitor signal to control the polarization state (i.e. a principal axis angle and the elliptic ratio) of the signal light to achieve the optimum state.