Relay apparatuses read the attribute values of data, i.e., numerical information representing measurement values, calculation values, control values, etc., from receive buffers in which data received from ECUs connected thereto is stored, and create trunk frames containing the numerical information and transmit and receive trunk frames via trunk lines. The relay apparatuses derive numerical information from trunk frames received from the other relay apparatuses and renew the numerical information on the data stored in their respective databases using the derived numerical information.