Upon selection of the frequency of a tone signal, a local oscillator generates a frequency signal having a shift frequency corresponding to the frequency of this tone signal. A frequency shifter generates a frequency-converted signal by shifting a frequency of the tone signal by a shift frequency of the frequency signal generated by the local oscillator. A BPF removes an image signal from the frequency-converted signal which the frequency shifter generated, and a normalizer normalizes the amplitude of the signal output from the second BPF. By this, even if the amplitude of the tone signal is small, the amplitude thereof is unified to a suitable magnitude. For this reason, even if there is a difference in the amplitude of the tone signal, the tone signal can be detected reliably and an false detection can be prevented.