A first optical splitter splits an input optical signal and outputs it to first and second optical paths. A second optical splitter outputs the optical signal from the first optical path to third and fourth optical paths. A third optical splitter outputs the optical signal from the second optical path to fifth and sixth optical paths. In the second optical path, 1-symbol delay element and π/4 phase shifter element are configured. In the fourth optical path, π/2 phase shifter element is configured. First and second adjuster circuits adjust the optical path length of the second and the fourth optical paths, respectively, by temperature control. A first optical coupler couples optical signals transmitted via the third and the fifth optical paths. A second optical coupler couples optical signals transmitted via the fourth and the sixth optical paths. Photodetectors convert the optical signals from the optical couplers into electrical signals.