PURPOSE: To avoid the system from being affected depending on the length of a transmission line by connecting a slave station device and a sub-slave station device with the digital transmission line so as to decrease the number of transmission lines between the slave station device and the sub-slave station device.

CONSTITUTION: Sub-slave station devices 31∼3m have one or more supervisory equipment 4, and its output state is converted into a digital signal prepared for a reply signal. A specific address is assigned to each sub-slave station device, each sub-slave station device supervises an address signal from a central device 1 via the slave station device 2m and digital transmission lines L1∼Lm, and when the own station is designated, the said reply signal is transmitted. Thus, the central device 1 calls sequentially the slave station devices 21∼2m and the sub-slave station devices 31∼3m by using the address signal and supervises the state of the supervisory equipment 4 accommodated respectively to each device by using the reply signal returned from the devices.