A communication system having a plurality of communication nodes connected by a communication bus is provided. The system allows at least one non-periodical data message to have a higher priority order than a periodical data message that is transmitted in response to a periodically-transmitted reference message from a master node ECU. In such a manner, a transmission of an urgent non-periodical data message has a higher priority than a transmission of the periodical data message. As a result, a wait time before a start of a transmission of the non-periodical data message is reduced.