PURPOSE: To improve the function as a supervisory device by providing a means for generating a fault identification signal in response to the kind of fault of the own station to each slave station and transmitting the said fault indentification signal to a master station to identify the kind of a fault of other stations.

CONSTITUTION: The system is constituted that the control of a signal switching circuit is performed by an I/O unit 5 and the control between a CPU1 and an MODEM3 when a fault occurs is performed independently by using the I/O unit 5. If a fault occurs in an MODEM interface unit 2, for example, an RS signal is made active and a rule signal of 011011011... is transmitted from the I/O unit 5 to an SD via the signal switching circuit 4. If a fault occurs in the CPU1, the RS is made active similarly and a rule signal of 100100100... is transmitted from the I/O unit 5 to the master station on the SD via the signal switching circuit 4. It is possible for the master station to detect the kind of fault occurred in any unit of a slave station by identifying a code of the rule signal for identifying fault generation.