PURPOSE: To simply eliminate a noise sound component such as howling sound by emphasizing the noise sound component and revising a filter coefficient of an equalizer into a coefficient representing a notch filter characteristic so as to eliminate the noise sound component.

CONSTITUTION: Coefficients a0-a2, b1, b2 set to coefficient multipliers 24-26, 30, 31 of a digital filter are changed under the control of a central processing unit 14 to emphasize of a prescribed band as a parametric equalizer and to attenuate a prescribed band as a notch filter. In this case, secondary IIR digital filters connected in 31-stages are used to form the parametric equalizer and the notch filter in which the center frequency changes in 31-stages at an interval of 1/3 octave. Then the gain of the equalizer at a prescribed frequency is increased by shifting the center frequency so as to emphasize the noise sound component and the filter coefficients of the equalizer are revised into coefficients representing the notch filter characteristic to eliminate the noise sound component.