PURPOSE: To prevent hawling by generating an intermittent wave which has a higher frequency than an audio frequency, and canceling a hawling component by the generated wave.

CONSTITUTION: An input wave from a microphone 1 is amplified by a preamplifier 2, whose output is supplied partially to a high-pass filter 6 via a by-pass, and then only a wave which frequency and period are equal to those of an intermittent wave generated by an oscillator 12 is outputted to a phase detector 7 and a level comparator 8. The phase detector 7 detects a phase difference between those two intermittent waves and inputs it to a phase corrector 10. On the other hand, part of the superposed wave of a power amplifier 4' is supplied to a variable amplifier 11, which is controlled by an output proportional to a level compared by the level comparator 8 to send its output to the phase corrector 10. The output of the phase corrector 10 is supplied to a differential amplifier 3 to cancel a hawling component from the preamplifier 2, thus preventing hawling.