PURPOSE: To make the adjustment easy without a band-pass filter by detecting accurately and stably only a signal having a specific frequency in the circuit which detects the AC signal having a specific frequency.

CONSTITUTION: When a frequency signal from an exchange is applied across a line and an earth, the frequency signal is extracted by the first filter FL1. It is selected furthermore by the second filter FL2 having a steep breaking characteristic and a narrow pass band and is converted to a DC voltage and is given to a reference voltage generation control circuit VSC. The control circuit VSC generates a reference voltage when the output voltage of a rectifying circuit REC exceeds the reference voltage. This reference voltage is compared with a comparison voltage corresponding to the output of the filter FL1, and the reference voltage is held in a certain value in accordance with this comparison output.