PURPOSE: To make the output voice of a 1st amplifier easy to hear by providing a 1st and a 2nd amplifier which select and amplify a 1st and a 2nd frequency band and a controller for the amplification factor of the 1st amplifier, and increasing the amplification factor of the 1st amplifier automatically when a noise including the 1st and 2nd frequency bands is generated at the periphery.

CONSTITUTION: A filter 4 and a filter 5 select only the 1st and 2nd frequency bands from the entire frequency band and output them. The 1st amplifier l amplifies the output of the filter 4 with the amplification factor controlled with the control signal of an amplification factor setter 6 and the control signal of a controller 3, and outputs the amplified signal. The 2nd amplifier 2 amplifies the output of the filter 5 with a previously set amplification factor and outputs the amplified signal. The controller 3 controls the amplification factor of the amplifier l so that the difference between or the ratio of the output levels of the amplifiers l and 2 becomes constant. Then the 1st frequency band is assigned to a voice band and the 2nd frequency band is assigned to the ultrasonic wave band which is outside the 1st frequency band and nearby to increase the voice output when the noise at the periphery of a listener is loud and decrease the voice output when the noise is small.