PURPOSE: To extract only a phase fluctuation component of an interference wave and the phase fluctuation of the desired wave by using a phase detection circuit detecting the phase fluctuation component in which a desired wave and an interference wave are overlapped.

CONSTITUTION: A limiter circuit 15 generates an n-order harmonic wave with respect to a reception signal subject to n-phase modulation and a band pass filter 16 is used to extract a frequency near the n-order harmonic frequency. The detection circuit is provided with a gate circuit 17 extracting a signal in the vicinity of an identification point and a circuit 18 generating a voltage according to the phase or frequency of the output of the circuit 17.