PURPOSE: To suppress a noise caused by mechanical vibration and impact of a reed filter, by detecting an output of the reed filter through a high-pass filter.

CONSTITUTION: A carrier wave modulated by a prescribed tone frequency signal is picked up by an antenna 11, and is led to a receiving part 12. The receiving part 12 demodulates the carrier wave and leads it to a low frequency amplifying part 13. An output of the low frequency amplifying part 13 is inputted to a reed filter 14, also passes through a high-pass filter 15, and is outputted from a level detector 25. In this case, frequency of a noise caused by mechanical vibration and impact of the reed filter 14 is half of nominal resonance frequency of the reed filter 14, and the noise is suppressed by the high-pass filter 15.