PURPOSE: To unnecessitate wiring of systems by judgement and to simplify constitution.

CONSTITUTION: A remote station 1 is constituted so that it has a function for transmitting the state of power voltage, and the state is transmitted to a master station 2 for a prescribed time when a power source of the remote station 1 is turned off by a lower source backup circuit. As a result, when the power source of the remote station 1 is turned off, and internal control voltage maintains a normal state for a while and becomes the state that data can be transmitted. In such a state, when address information of the own station is received from the master station 2, the state that the power source is abnormal is sent out as data to the master station 2 through a transmission line 3 from a power voltage monitoring circuit in the remote station, by which the master station 2 can judge whether the remote station 1 is in the state that the power source is turned off, or the remote station 1 is faulty.