PURPOSE: To reduce output noise in the vicinity of an output signal and to improve the output S/N by interposing a band stop filter blocking a frequency at a band width of a constant band amplifier around a frequency of an input signal between a local oscillator and a mixer.

CONSTITUTION: A local oscillation signal enters a band stop filter 4 and part (at frequencies 23) of output noise 21 caused at a band width coverage of a constant band amplifier 3 around an input signal spectrum 30 is suppressed by the band stop filter 4. Thus, a part 24 corresponding to the band width of the constant band amplifier 3 in the output noise 22 of the frequency conversion circuit is reduced around an output signal spectrum 31 subject to frequency conversion. Thus, the output noise in the vicinity of the signal spectrum 31 of an output signal 11 is suppressed and the output S/N is improved.