PURPOSE: To prevent a higher harmonic component from remaining in an output signal.

CONSTITUTION: The low-frequency sound part, of an input audio signal is extracted at a first low-pass filter 1 and turned to a rectangular wave at an amplitude limiting circuit 2 and afterwards, the frequency is divided into the 1/2 frequency by a frequency divider circuit 3. On the output side of the frequency divider circuit 3, a variable frequency interdigital filter 8 is provided with a variable delay circuit 9, the 1/3 time of the basic wave component in the output of the frequency divider circuit 3 is measured by a delay control circuit 11, delay control is executed to the variable delay circuit 9 so that the delay time can be the said 1/3 cycle, and a third higher harmonic wave is removed while passing the basic wave component as it is. The fifth higher harmonic wave in the output of the variable frequency interdigital filter 8 is removed by a second low-pass filter 4, only the basic wave component is inputted to a voltage controlled amplifier 5 and outputted to the rear step after providing an amplitude change similar to the amplitude change at the low-frequency sound part of the input audio signal based on the detection output of an envelope detection circuit 6.