To provide a technology that dynamically sets a relay path when a structure of a communication system is changed.SOLUTION: Relay devices 20, 60, 70, and 80 by one embodiment of this disclosure, are relay devices for relaying communication messages communicated between communication nodes 10, 12, and 14, and are respectively equipped with reception processing portions, transmission processing portions, and relay processing portions. The transmission processing portion transmits a communication message received by the reception processing portion. When the reception processing portion receives a relay request message notifying a communication message to be relayed and a relay destination of the communication message, and a transmission advertisement message notifying the communication message to be relayed and a relay source of the communication message, the relay processing portion sets a relay path indicating the communication message to be relayed, the relay source, and the relay destination, and relays the communication message communicated between the communication nodes and received from the relay source by the reception processing portion, from the transmission processing portion to the relay destination on the basis of the relay path.SELECTED DRAWING: Figure 4