Sensors detect parameters. Control units control vehicle equipments in accordance with the detected parameters. In each of the control units, at least one of the detected parameters and internally produced data is converted into serial data as communication data, and serial data is converted into other form data. A transmission medium connects the control units via a serial data link. Serial interfaces are associated with the control units respectively. Each of the serial interfaces includes a connection terminal connected to the transmission medium, an input terminal and an output terminal both connected to the associated control unit. Each of the serial interfaces inputs serial data from the associated control unit via its input terminal and outputs the inputted data to the transmission medium via its connection terminal. Each of the serial interfaces inputs serial data from the transmission medium via its connection terminal and outputs the inputted data to the associated control unit via its output terminal.