PURPOSE: To improve the S/N of a filter output and to prevent a malfunction in a tone detection circuit from being generated, by providing a band-pass filter at the output of a low-pass filter.

CONSTITUTION: An input tone passes through a pre-low-pass filter 1, the low-pass filter 2, and the band-pass filter 7. And it passes through a limiter 3 whose detecting sensitivity is decided by a limiter control input 5 and a detector 4, and an output 6 which represents the detection of a preset tone can be obtained. The input of the limiter 3 includes no tone in a settable frequency band as a noise. In such a way, it is possible to prevent the malfunction in a preset tone detection circuit from being generated.