A data communication network system which enables an intended unit to determine a start point for a transmission schedule in communication is provided.In a data communication network system in which a plurality of electronic control units perform network communication through a communication line based on a time triggered communication protocol, a control section for previously putting the communication line in a communication inhibited state, and canceling the communication inhibited state at a predetermined time is included, and a predetermined one unit among the plurality of electronic control units transmits a message for synchronization, in a communication time, with the other units thereamong immediately after the communication inhibited state is cancelled.