A first interferometer comprises a first delay element and a first phase shift element, and a second interferometer comprises a second delay element and a second phase shift element. The amounts of phase shift in the first and second phase shift element are zero and π/2, respectively. A first photo detector comprises first and second photodiodes connected in parallel, and a second photo detector comprises third and fourth photodiodes connected in series. The first photodiode is provided with a first optical output of the first interferometer, the second photodiode is provided with a first optical output of the second interferometer, the third photodiode is provided with a second optical output of the first interferometer, and the fourth photodiode is provided with a second optical output of the second interferometer. A signal process circuit recovers transmitted data based on output signals of the first and second photo detectors.