A relay device directly connected to end nodes in an in-vehicle network transmits a failure detection packet to each end node. When each end node can receive the failure detection packet, each end node transmits a response to the relay device. When the relay device receives the response, the relay device transmits another failure detection packet. The end node determines that the in-vehicle network is normal when the value of the previous failure detection packet does not match the value of the other failure detection packet, and determines that the in-vehicle network is not normal when the value of the previous failure detection packet matches the value of the other failure detection packet.