The relay devices store data received from an ECU connected to each of the relay devices in the first reception buffers, respectively. During a synchronizing period, the relay devices perform transmission and reception by mainline frames including attribute value groups mutually received from the ECUs. Data read out from the databases are transmitted to the ECUs. The databases are synchronized with one another only when the relay device receives the final frame from the relay device which received all the mainline frames and issues COMMIT.