PURPOSE: To simply and surely detect a signal by providing two filters, a comparator circuit, a decision circuit, a time protection circuit and a function for utilizing its output signal as a suppression signal and a band stop signal to the in-band one-frequency detection circuit.

CONSTITUTION: A linear PCM signal 10 received from an input terminal 1 is inputted to a one-frequency band pass 'digital filter BPF 2, from which only a 1st signal component is extracted and inputted to a frequency band pass digital filter BPF 3, from which.a 2nd signal component with a 1st signal component eliminated therefrom is extracted. The two extracted signal component levels are compared by a comparator circuit 5, which outputs a signal representing whether difference of the both levels is larger or smaller than a set comparison value. When a difference of levels of both the signals is larger than the setting value, a decision circuit 6 outputs a 1st digital code and when a difference of levels of both the signals is smaller than the setting value, the decision circuit 6 outputs a 2nd digital code. Thus, an in-band one-frequency component is not mis-recognized from a frequency component included in a voice signal.