PURPOSE: To easily detect the state of a transmission line and the power supply working state for an equipment set at the side where a trouble is detected at the trouble detecting side, by securing a short circuit state within the equipment between the reception and transmission lines in case the power supply is cut for the equipment set at the side where the trouble is detected.

CONSTITUTION: If a power supply 6B of an equipment B set at the side where the trouble is detected by some reason, a transmission line switching circuit 5B switches a transmission line connected to the α side in a normal state to the β side. Thus an MPU 1B is separated from the transmission line and the transmission signal received from an MPU 1A of a trouble detecting equipment A is returned by the circuit 5B and sent back to the MPU 1A. At the same time, the received data are always monitored by the MPU 1A and it is judged that the equipment B has the cut-off of the power supply in case the coincidence is obtained between the received data and its own data. At the same time, it is also known that the transmission line has no abnormality.