A plurality of zones is formed in a communication network on the vehicle, and a communication path in a loop form is formed in each zone by zone trunk lines and a backup line. The backup line is connected by a switch only at the time of disconnection so that the loop is not closed. At the time of disconnection detection, an instruction is automatically given to rewrite content of routing maps on zone ECUs and a central gateway. Since each node preferentially selects a communication path that bypasses a disconnection portion from the start of communication by rewriting the content of the routing maps, it is possible to avoid an increase in communication delay. An inter-zone backup line connecting a plurality of zones is provided to enable the use of a new path across the plurality of zones at the time of disconnection.