A certain ECU transmits a reference message for requiring the other ECUs to transmit data. After transmission of the reference message, each of all the ECUs transmits priority information of its transmit message onto a communication bus, and then detects whether some priority information transmitted from the other ECUs has a higher priority than its own transmitted priority information. If there is detected no priority information of a higher priority than its own transmitted priority information, it transmits a message associated therewith, and then is prohibited to transmit data of the same priority until receiving a next reference message.