This invention provides a relay connection unit that relay message and make sufficient number of branches to be able to connect a plurality of ECUs. The relay connection unit includes: reception connectors to which a plurality of insertion connectors of communication lines can respectively be connected; a branch connection unit that divides the reception connectors into a plurality of groups, the branch connection unit branch-connecting the communication lines through the reception connectors belonging to the respective groups; a plurality of vehicle-mounted LAN communication units respectively connecting to each of the groups of the reception connectors via the branch connection unit, which are configured to perform transmission/reception of messages; and a relay processing unit that relays an incoming message received via one of the vehicle-mounted LAN communication units to another of the vehicle-mounted LAN communication units of another relay connection unit in accordance with a relaying destination.