To warn the presence or absence of tapping by converting a voice signal of a reception frequency with waveform data and comparing the data with waveform data converted by an optional sound source circuit via a voice input terminal.

An operating frequency detection section 2 sweeps a reception frequency from an antenna 1 over a prescribed frequency range to demodulate a voice signal and a detection signal is fed to a control section 4 and the voice signal is fed to a voice data extract section 3. Furthermore, an optional voice signal from a sound source circuit 7 is outputted from a speaker 5 via the control section 4 and converted into waveform data. Moreover, the voice signal received by a voice input terminal 8 is converted into waveform data by the control section 4. The control section 4 compares the waveform data from the voice data extract section 3 with the waveform data converted by the sound source circuit 7 via the voice input terminal 8 and when the both are coincident, the operating frequency is discriminated to be an operating frequency of a tapping device and the result is warned from a display device 6 and the speaker 5. Thus, the presence or absence of the tapping is recognized.