A vehicle-mounted Local Area Network (LAN) system includes vehicle-mounted LANs in each of which an electronic control unit is connected to a communication line to send and receive messages. The LAN system also includes a relay connection unit that relays the messages between the vehicle-mounted LANs. Each electronic control unit has a reception port for transmitting a reception confirmation signal to the relay connection unit in response to a received message. The relay connection unit has a relay management device that determines, based on the presence or absence of the reception confirmation signal responsive to the message relayed to each of the vehicle-mounted LANs, whether or not a relay is required, and that then updates relay processing information stored in a storage device.