PURPOSE: To perform satisfactory transmitting and receiving and to prevent. side tone by detecting the level of line current, making gain control of a receiving amplifier and a transmitting amplifier to obtain the most suitable level for signal transmission, and at the same time, controlling the level of the receiving amplifier detecting the level of transmitted signals at the time of transmission.

CONSTITUTION: When the distance of transmission is large,the level of line current is lowered due to large line resistance. Therefore, the gain of amplifiers 1, 11 is controlled to high gain to make the level of line current high and make smooth transmitting and receiving possible. At this time, the signals that leak from transmitted signals to received signals are amplified largely by the receiving amplifier 1 that became high gain, and accordingly, the side tone becomes large. However, due to the presence of the second gain controlling circuit 13, the side tone is suppressed. the presence of transmitted signals sent out from the transmitting amplifier 11 is detected by a second gain controlling circuit 13 to lower gain of the receiving amplifier 1. Thus, the amplification of the leak signal as side tone is suppressed, and the sound generated from a speaker is made low.