PURPOSE: To enable highly accurate detection of a phase difference between an optical signal and an electrical signal by computing a product of a voltage signal and the optical signal with a photoconductive type photodetector directly to measure a value as the results of the operation averaged hourly.

CONSTITUTION: An AC component of a voltage signal inputted from a voltage application section 200 is applied to a photoconductive type photodetector 110. When an optical signal is incident into the photoconductive type photodetector 110 to which the voltage is applied, a current corresponding to a value of a product of a voltage signal and the optical signal flows to the photoconductive type photo detector 110. The resulting current signal is inputted into a current- voltage conversion section 120 through a choke coil and converted to a voltage signal to be outputted. The voltage signal contains an AC component more than can be removed with the choke coil. A lowpass filter 130 inputs the voltage signal to operate a hourly averaging thereof and a DC voltage is outputted. This DC voltage value coincides with a value W of a phase difference output.