PURPOSE: To reproduce the frequency characteristic to a state close to a flat characteristic in a listening sense even when a level of an acoustic signal is small by extracting part of a frequency band of the acoustic signal and increasing/decreasing the level of the part of the frequency band corresponding to the extracted level.

CONSTITUTION: While an acoustic signal is inputted to a tone control circuit 15b, part of the signal is inputted to a low pass filter 11 and a low frequency component of the acoustic signal is extracted. The extracted low frequency component is converted into a DC voltage by a waveform shaping circuit 12 and inputted to an adder 14 together with an output voltage of a reference output generator 13. The adder 14 consists of a differential amplifier and an output corresponding to a difference between two input signals is obtained, The output of the adder 14 is inputted to a voltage control circuit 15a, in which a voltage required to correct the low frequency component is set. The voltage correcting the low frequency component is inputted to a tone control circuit 15b, from which an acoustic signal with the frequency characteristic whose low frequency component is emphasized is obtained.