PURPOSE: To enable silencing over a wide band by making a frequency characteristic to be flat so as to get the equal sensitivity at each frequency by arranging an inverse filter in the rear step of a detection microphone so as to make flat the frequency characteristic of the detection microphone composed of plural microphones.

CONSTITUTION: Inverse filters 7 and 13 are provided to make the frequency characteristic of an entire detection microphone 2 so as to get the equal sensitivity at each frequency, and a control means 8 is provided to generate the inverted audio signal of an inverse phase and the equal amplitude to noise in a propagation path 1 based on a noise signal while receiving this noise signal detected by the detection microphone 2 through the inverse filters 7 and 13. In this case, the frequency characteristic of the detection microphone 2 itself lowers the detection level much more in a low frequency area and since the noise signal is equalized by the inverse filters 7 and 13, however, the frequency characteristic of the detection microphone 2 is practically turned to the equal sensitivity to each frequency. Thus, since the noise advanced in the propagation path 1 can be exactly detected even in the lower frequency band, the silencing is enabled over the wide band for the detected component.