When detecting a fault of a master control device, a slave control device refers to master-device-line information, and obtains line information to establish a connection with the other master control device. The slave control device transmits connection request information to the master control device extracted from the other master control devices. The other master control device determines whether or not it is connectable with the slave control device that has transmitted the connection request information, and transmits a determination result as connectability information. The slave control device switches a communication line to the other master control device when the received connectability information indicates the connectable status.