PURPOSE: To detect both transmissions simultaneously transmitted from a two- way circuit, by installing a timer and an electric current monitoring circuit which are started to work at the time of the trunk transmission.

CONSTITUTION: When a trunk transmission is caught at the exchange side, relays A and B start to operate and the opposite station side is started to operate. Since a relay RV also is kept operate at this time, a relay E is set to the operalbe condition by the earth and battery of the counter station, and is kept set to the monitor the DC electric current of the circuit. On the other hand a relay T which is delayed a little behind the operation of the relay B is operated and, when the DC current flows through the circuit, a relay RL does not work because the relay E is kept operates. However, when the trunk of the counter station is set to the transmitting condition almost at the same time, the DC current does not flow through the circuit and the relay E does not work, and, therefore, the relay RL works. In this way, the relays B and RV are forcedly restored to the original condition, and the restoration information is transmitted to a control section through a terminal 5.