An optical receiver comprising: a polarization diversity optical mixing part arranged to mix a received phase-modulated optical signal and local oscillator light in at least two kinds of phases and extracting at least two optical signals corresponding to each light phase for demodulating the phase-modulated optical signal and arranged to extract real part and imaginary part of each of orthogonal polarized wave components of the phase-modulated optical signal; a photoelectric conversion part (11-1, 11-2, 11-3, 11-4) arranged to convert the at least two optical signals obtained in the optical mixing part into electric analog signals; an analog-to-digital conversion part (13-1, 13-2, 13-3, 13-4) arranged to convert the electric analog signals obtained by the photoelectric conversion part into digital signals; a control part arranged to process the digital signals obtained by the analog-to-digital conversion part thereby detecting a light phase difference of the optical signals of the optical mixing part and to supply a signal for correcting the light phase difference of the optical signals such that the correlation between real part and imaginary part becomes zero to control the optical mixing part so that the light phase difference is corrected to value zero or close to zero; and a decision part arranged to discriminate between the digital signals, wherein the control part detects a phase difference between each digital signal before discrimination by the decision part and each digital signal after discrimination by the decision part and uses a control signal corresponding to the magnitude of a variation of the phase difference for phase shift control of the optical signals of the optical mixing part. a polarization diversity optical mixing part arranged to mix a received phase-modulated optical signal and local oscillator light in at least two kinds of phases and extracting at least two optical signals corresponding to each light phase for demodulating the phase-modulated optical signal and arranged to extract real part and imaginary part of each of orthogonal polarized wave components of the phase-modulated optical signal; a photoelectric conversion part (11-1, 11-2, 11-3, 11-4) arranged to convert the at least two optical signals obtained in the optical mixing part into electric analog signals; an analog-to-digital conversion part (13-1, 13-2, 13-3, 13-4) arranged to convert the electric analog signals obtained by the photoelectric conversion part into digital signals; a control part arranged to process the digital signals obtained by the analog-to-digital conversion part thereby detecting a light phase difference of the optical signals of the optical mixing part and to supply a signal for correcting the light phase difference of the optical signals such that the correlation between real part and imaginary part becomes zero to control the optical mixing part so that the light phase difference is corrected to value zero or close to zero; and a decision part arranged to discriminate between the digital signals, wherein the control part detects a phase difference between each digital signal before discrimination by the decision part and each digital signal after discrimination by the decision part and uses a control signal corresponding to the magnitude of a variation of the phase difference for phase shift control of the optical signals of the optical mixing part.