PURPOSE: To perform accurate demodulation by comparing the phase difference between input and output signals of a narrow band pass filter having a variable center frequency which a converted signal passes and applying the comparison output to the narrow band pass filter through a low pass filter to make said phase difference zero.

CONSTITUTION: A reception signal 1 is inputted to a remodulating part 201 and an orthogonal phase demodulating part 202 through a preamplifier 101. The demodulated signal is inputted to the remodulating part 201 to produce a carrier signal 18. The signal 18 becomes the reference reproduced carrier signal 6 of the demodulating part 202 through an AFC part 203 to form a loop. The signal 18 is multiplied in a multiplier 115 by the signal supplied from an oscillator 112, and the output of the multiplier 115 is supplied to a multiplier 120 and a narrow band BPF 117 through a BPF 116. The output of the BPF 117 is supplied to a variable phase shifter 119 and a multiplier 123 through a limiter circuit 118, and the phase comparison output of the multiplier 120 is supplied to the BPF 117 through an LPF 121 to change the center frequency of the BPF 117. Thus, the frequency of the AFC loop is made constant to perform the accurate demodulation.