A bus communication system capable of determining an order of priorities
based on IDs of transmitting nodes is provided. A communication unit
writes its own node ID as a transmitting node ID into an arbitration
field of a message packet that is to be transmitted from the
communication unit to a bus, writes a node ID of a destination node into
an auxiliary field defined at a predetermined location in the message
packet, and transmits the message packet to the bus. A control unit
rewrites the message packet transmitted from the communication unit by
replacing the node ID written in the arbitration field with the node ID
written in the auxiliary field, and transmits the rewritten message
packet to the bus. Another communication unit, or the destination node,
receives the message packet existing on the bus based on a match between
its own node ID and the node ID indicated in the arbitration field
rewritten by the control unit.