PURPOSE: To exert no influence on a normal system because an interface to other device is reduced, and separated completely by a switching relay, by providing a fault detecting part on the own device, and generating a switching signal.

CONSTITUTION: When a duplex device "0" system 200 is operated, if an alarm is raised to an alarm line 201, an alarm detecting part 400 knows that the duplex device "0" system 200 has become faulty. Therefore, contacts 501, 502 are opened by operating a detaching relay 500, and the duplex device "0" system 200 is detached from signal lines 100 and 700. At the same time, the alarm detecting part 400 stops the duplex device "0" system 200 by using a control line 401, and on the other hand, sends an operating instruction to a duplex device "1" system 300 by using a control line 402. When the alarm from the alarm line 201 has gone, the detecting part 400 stops the relay 500, closes the contacts 501, 502, and resets them to a normal state, but the state of the control lines 401, 402 is not changed, therefore, the duplex device "0" system 200 stops, and the duplex device "1" system 300 continues its operation.