On-vehicle communication system

An on-vehicle communication system in which a relay device capable of relay of communication and path selection according to a destination of communication is provided at each of a plurality of positions, for which branching off is possible, on a wire harness including a trunk line of a communication line,

• wherein each of a plurality of the relay devices comprises:
• a failure detection part that detects disconnection or decrease in communication quality;
• a routing map in which a plurality of types of path information determined in advance is held in correspondence to presence or absence of a failure and each failure portion;
• and
• a path control part that controls switching of the routing map,
• wherein the trunk line of the communication line is formed in a state where it is possible to selectively use a plurality of types of paths physically or logically,
• wherein when the failure detection part of any relay device detects failure occurrence, the path control part in the relay device whose failure detection part detects the failure occurrence instructs another relay device to change a path, and
• wherein when the failure detection part of any relay device receives an instruction of path change from another relay device, at least the routing map in the relay device whose failure detection part receives the instruction of path change is switched in accordance with a failure occurrence portion.