To transmit a signal light by an optical polarization modulation system without requiring polarization control on a receiving side.

Signal light is obtained by performing optical polarization modulation for making polarization orthogonal to the polarizing state of light preceding by one bit when a transmission signal is '0' and making the same polarization as the polarizing state of light preceding just before by one bit when a transmission signal is '1' and is transmitted, and a received signal light is synthesized with the signal light preceding by one bit. In the case of mutually orthogonal polarization, they are outputted to two output ports without interference and when they are in the same polarizing state, they are outputted to one of the two output ports by interference. Then, the signals of the respective output ports are transduced to electric signals and the difference signals of both of them are outputted as a demodulation signal with respect to a transmission signal.