Switching hubs such as relay devices capable of path control are arranged at a plurality of branch positions respectively on a wire harness including a trunk line of a communication line. Each of the relay devices includes a failure detection part, a routing map, and a path control part. The trunk line of the communication line is formed to make it possible to use a plurality of types of paths selectively. When the failure detection part in any relay device detects a failure, an instruction of path change is given to another relay device therefrom. When any relay device receives an instruction of path change from another relay device, the routing maps in the relay device receiving the instruction are switched according to a failure occurrence portion. A VLAN is constructed on an Ethernet communication network, and logical assignment of the VLAN is controlled by a central gateway.