A relay connection unit which is connected to a first communication line of a first communication protocol for communicating a first message via time-divided slots and a second communication line of a second communication protocol for communicating a second message with the ID and converts the communication protocols to relay the first message and the second message. The relay connection unit is characterized by converting a first storage table in which the ID of the second message is assigned for each time-divided slot for the communication of the first message and the second message into the first message and having processing section for transmitting the converted first message to the first communication line via the time slots assigned in the first storage table.