To improve an S/N in a high band by generating a first oscillation bias signal inversely proportional to the level of a signal in the high band of an input audio signal and generating a second oscillation bias signal of a specified level in middle and low bands.

A multiplication part 140 inputs a detected output signal of a detection part 120 and a bias sine wave output signal of an oscillation part 130, and multiplies both signals by each other to output it to a differential amplifier part 150. Then, the output signal of the multiplication part 140 from the high band becomes the signal multiplying a peak value of an input signal by a bias sine wave oscillation signal, and the output signal from the middle, low bands becomes the bias sine wave oscillation signal with a fixed DC level. The differential amplifier part 150 inputs the output signal of the multiplication part 140 and the oscillation signal of the oscillation part 130, and operates a difference between the signal multiplying a high frequency input signal by the oscillation signal and the oscillation signal to output it. Thus, the output of the amplifier part 150 becomes the oscillation signal having the level inversely proportional to the level of an input audio signal of the high band.