In a connection device connected to a network mounted in a boat, a protocol of a connection device connected to a network mounted in a boat is identified based on a communication ID in a certain message transmitted through the network. A device protocol is determined in accordance with the protocol of the network. Data communication in the network is performed in conformity with the device protocol. Accordingly, when the connection device is connected to the network, the connection device automatically identifies a protocol of the network and operates appropriately. Therefore, upward compatibility is maintained and usability is improved.