PURPOSE: To eliminate the effect of an unnecessary signal component such as noise by providing a switch control circuit switching selectively a transmission side signal switch circuit or a reception side signal switch circuit depending on the discrimination output of a signal discrimination circuit and a comparison output of a comparator into signal passing state or signal cut-off state.

CONSTITUTION: The reception side signal discriminating circuit 36 discriminates whether a reception signal at a reception system 6 is a required signal component or an unnecessary signal component, and when the required signal component exceeds a prescribed level, e.g., a sound signal component exceeds a prescribed level, then a high level discriminating output is generated. A comparator 30 compares the levels of the signals fed from a transmission system 4 and a reception system 6 and for example, when the transmission signal level is larger than the reception signal level, a positive comparator output proportional to the difference is fed to the switch control circuit 32 and the mode is brought into the transmission mode. Conversely, when the reception signal level is larger than the transmission signal level, the mode is brought into the reception mode and a negative comparator output proportional to the difference is fed to the switch control circuit 32.