Objective SAE defines the levels of automatic driving system, e.g. in case of level 3 system, the system has a responsibility for the driving in ordinary conditions, and a driving task change from the system to the driver in an emergency case. When the driving change from the system to driver, it is necessary to take a safety transition time. The regulation of the Automatic Commanded Steering Function discusses the transition time in the United Nations. The driver takes a relaxed driving position using the automatic driving system. This research aims to investigate the relationship between the driving position and the transition time. Experiment The experimental conditions was the changing of driver’s posture, and the condition was (1) backrest angle, (2) seat position, (3) foot condition and (4) arm condition, total condition was 30 patterns. In addition, the condition which the driver did not gaze the front was simulated by the driver controlled the tablet computer. The experiment used two scenarios for measuring the driver’s braking and steering action. The scenarios were that the truck dropped a cargo in front of the host vehicle on highway. And the acoustic and optical warning was presented to driver when the automatic driving change to the manual driving. Conclusion In case of the driver’s braking and steering action, this research analyzed the relationship between the driving position and the transition time. The reaction time was the analysis item. As a result, the driver’s posture of foot condition was the most affected the reaction time in the case of braking action. In case of steering action, the arm condition which is resting driver's chin on driver's hands was the most affected. Using the automatic driving system, this research clarified the difference of driving characteristics caused by the relaxed driving position of driver.