PURPOSE: To decrease a detection error of an out-band signal, by providing an amplitude limiter on the prestage of a band-pass filter.

CONSTITUTION: An in-band signal which has been applled to an input terminal 1 is inputted to a band-pass filter 2 by an amplitude limiter 5, as it is in case when it is smaller than its amplitude limiting value A, and by being limited by this limiting value in case when it is larger. A signal which has passed through the band-pass filter 2 is detected as an insignificant signal in case when it is smaller than a threshold level B of an amplitude detector 3, and as a significant signal in case when it is larger. Accordingly, since a frequency characteristic of a maximum detection sensitivity becomes sharp, a detection error is extremely decreased.