In a master device, an address for a transmission origin and an address of an ultimate transmission destination, included in a message from an end device, are stored, as a pair, as proxy information, and the address of the transmission origin included in the message is overwritten with the local communication address, to form a proxy message to the destination device. Moreover, the master device performs a search for a round-trip route to the destination device and sends the proxy message to the destination device following the outbound route that has been searched. The destination device, upon receipt of a proxy message, sends back a message to the master device following the inbound route that has been searched by the master device. The master device uses the proxy information to send back, to the end device, the message from the destination device.