PURPOSE: To obtain the optimum input level automatically, by performing the stoppage of the estimated arithmetic operation of an acoustic echo canceller and applying automatic adjustment on the input level of an A/D converter when the overflow of the A/D converter is generated.

CONSTITUTION: A voice signal from a microphone is amplified to a required level at an amplifier 6, and is inputted to the A/D converter 7, and when the overflow of the converter is generated, it is detected by a maximum amplitude value detection circuit 71. The detection circuit is outputted from a stop signal output terminal 76 to stop the estimated arithmetic operation of the acoustic echo canceller, and deterioration in an estimated value is prevented from being generated. Also, sample calculation for the generating frequency of the maximum amplitude value is performed at a calculator 72 by counting the number of times of generation within prescribed sample length at every operation, and the frequency is outputted to a gain controller 73. The gain controller generates a level control output so as to reduce the difference of the permitted frequency of the stoppage of the estimated arithmetic operation set from a parameter input terminal 74 by the output value of the calculator 72, then, the gain of the amplifier 6 can be adjusted.