An optical mixing part mixing a received optical signal and local oscillator light in at least two kinds of phases and extracting at least two-system optical signals corresponding to each light phase; a photoelectric conversion part converting the at least two-system optical signals obtained in the optical mixing part into electric analog signals; an analog-to-digital conversion part converting the electric analog signals into digital signals; and a control part processing the digital signals thereby detecting a light phase difference between the respective systems in the optical mixing part and supplying a signal for correcting the light phase between the systems to the optical mixing part to control the optical mixing part so that the light phase difference becomes to zero or close to a desired value when the light phase difference has a shift from the desired value.