A communication network in which a plurality of terminals are connected to a common bus is a vehicle-mounted communication system which connects a plurality of ECUs though the bus, transmits messages at predetermined communication intervals between the ECUs, and receives the transmitted messages. The messages transmitted/received between the ECUs through a CAN communication line are divided into a plurality of groups such as a first group, a second group to an Nth group according to the type of message information. Each of the ECUs is provided with a transmission timing operation part for computing the transmission timing for each message group to be transmitted. The transmission timing operation part adjusts so as to bring the transmission timing of message belonging to the first group transmitted from its own ECU closer to the transmission timing of message belonging to the first group received from another electronic control unit.