An electronic control unit has two microcomputers. Each microcomputer has a data buffer storing data to be transmitted in every 8 ms, a data buffer storing data to be transmitted in every 16 ms, and another data buffer storing data to be transmitted in every 16 ms and being different from the data buffer. The microcomputer transfers at the transmission timing of every 8 ms data in the data buffer to the transmitting buffer, while it transfers alternately the data in the data buffer and the data in the data buffer to the transmitting buffer. The microcomputer also transfers an ID that indicates content of the present transmitting data to the transmitting buffer.