An optical transmitter splits a linear polarization optical carrier into a TE wave and a TM wave. One of the TE wave and the TM wave is phase-modulated with a transmission data so as to generate a phase-modulated signal. The phase-modulated signal and the other one of the TE wave and the TM wave are coupled with a linear polarization and output into an optical transmission line. An optical receiver splits a light input from the optical transmission line into the TE wave and the TM wave. The TM wave or TE wave is converted into the TE wave or TM wave to interfere with the TE wave or TM wave. The interfered signal light is converted into an electric signal for restoration of the data.