To prevent transmission audio signals from being interrupted by eliminating echo control, when it is discriminated an audio input signal level exceeds a prescribed level.

At the time of initial state, namely when a received audio signal detection part 4 does not detect any received audio signal, a transmission audio signal selecting part 5 selects an audio input signal (a) and when the received audio signal detection part 4 detects the received audio signal, the transmission audio signal selecting part 5 switches the transmission audio signal from the audio input signal (a) to an output signal (b) of a level drop part 3. Moreover, when a voice is inputted to a microphone 1 and a reset signal (d) is outputted from a digital logic IC 6, the reset received audio signal detection part 4 forcedly switches the transmission audio signal to the audio input signal (a). Thus, the transmission audio signal is transmitted without lowering its level, and no phenomenon, such as interrupting the transmission audio signal, occurs.