To automatically detect and remove howling by amplifying an inputted voice, forcedly generating howling, detecting a frequency and attenuating it by an adaptive filter so as to extend the limit of the sound volume of a speaker.

The inputted voice from a microphone is amplified to a voice processable level by means of a microphone amplifier 1, serially connecting operational amplifiers A1 and A2 to forcedly generate howling. Next, a howling frequency is detected by a filter amplifier with a detection circuit 2, incorporating the notch filter of an attenuating characteristic obtained by dividing an audible frequency band into nine parts by each octave. In addition, a notch filter for attenuating this howling frequency band is selected. A buffer amplifier 3 corrects the sound volume before and after detecting howling, by means of an amplifier with the amplitude switch of the operational amplifier A5 to ultimately attenuate to a microphone inputting level. Thereby the limit for the sound volume of the speaker is magnified to automatically detect and remove howling.