A filter conducts the round trip by using the return optical signal that has been shifted in frequency, and measures the transmission optical signal and the return signal in phase by the principle of the Michelson interferometer at the same time, independently, and splits the two optical signals. A polarization state in which transmission and reception optical signals within an optical phase shifter which enters one route of the two optical signals are made orthogonal to each other is provided, to thereby distinguish the transmission and reception signals of the round trip from each other. The light is allowed to pass the shifter in incoming and returning to remove the polarization rotation of the shifter by using the reversibility of the light. Then, the phases of the transmission signal and the return signal are measured and synchronized with each other to conduct the transmission phase compensation.