PURPOSE: To receive the sound of a signal level to overcome a external noise through a receiver by gain-controlling automatically a received sound signal level in direct proportion to the magnitude of the external noise.

CONSTITUTION: The external noise in which a moving body 1 during communication is placed is collected by a microphone 7, and is amplified by a noise amplifier 8, and an automatic amplification gain control circuit 9 makes the amplification gain of the received sound to pass through a received sound amplifier 6 change automatically in direct proportion to an inputted noise signal level, and transfers it to the receiver 10. Accordingly, at the receiver 10, the received sound amplified in direct proportion to the magnitude of the external noise is obtained. Thus, even at the place where the magnitude of the external noise in which the moving body 1 is placed changes violently, the sound of the remote party of communication can be heard without being disturbed by the external noise.