PURPOSE: To display a noise reduction effect by an active noise controller maximally by improving the stability in control.

CONSTITUTION: Whether the noise reduction control falls into a divergence state or a sound increment state or not is judged by monitoring driving signals y1-y4 outputted from an adaptive digital filter 13, and when the fact that the noise reduction control falls into such a state is judged, the process specifying that any of microphones 8a-8h causes it is executed. Then, when the cause is specified, the input gains of remaining noise signals e1-e8 outputted by the microphones 8a-8h having the cause are made '0', and the microphones 8a-8h are cut off from a micro processor 16, and the noise reduction control is continued in the state of cutting off the microphones 8a-8h having the cause. When no cause is specified, the noise reduction control is interrupted.