Disclosed is a method of establishing a network topology capable of carrying out a relay transmission among sub-networks in a backbone network having the plural sub-networks composed of at least one device. The method includes each of sub-master devices controlling communications in the respective sub-networks transmitting an ID request message to other sub-master devices; transmitting a response message from the other sub-master devices to the sub-master device having transmitted the ID request message; each of the sub-master devices assigning an ID to the sub-master device having no ID among the sub-master devices having transmitted the response message; and forming a network topology in accordance with whether or not the response message has been transmitted, and in accordance with the order of ID assignments, after the ID is assigned to all the sub-master devices in the backbone network. The network topology enables reliable communications among the respective sub-networks.