PURPOSE: To attenuate an image frequency sufficiency and reduce insertion loss without using any inexpensive high-frequency BPF by making a frequency (phase) comparison between an intermediate frequency and a reference frequency and feeding the frequency (phase) back to a mixer through an LPF and a voltage-controlled oscillator.

CONSTITUTION: An intermediate frequency signal has its frequency (phase) compared with the output of a reference oscillator 7 of oscillation frequency fS by a phase comparator 8 and a frequency (phase) difference signal is generated. This frequency (phase) difference signal is inputted to the LPF 9, which outputs a DC voltage proportion (or in inverse proportion) to the frequency (phase difference. The oscillation frequency of the VCO 10 is controlled with the DC voltage. The LPF 9 outputs the DC voltage proportional to -{(fv'-fp)-fs} or -{(fp-fv')-fs}, where fv' is the oscillation frequency of the VCO 10 when an image frequency fp is inputted. Then the oscillation frequency of the VCO 10 is controlled with this DC voltage. Consequently, a feedback loop 16 becomes a positive feedback loop and is not locked and neither (fv'-fp)=fs nor (fp-fv')=fs holds, so that no signal is received at the image frequency fp.