It is said that when riding in the autonomous vehicle, the number of people who develop motion sickness increases. One of the reasons is that it is difficult for the passenger to predict behaviors of the autonomous vehicle and the head acceleration in the lateral direction becomes large by changing from driver to passenger. So, we propose a method to reduce the head acceleration in the lateral direction with the seat tilt device. The head acceleration in the lateral direction is compared and evaluated with or without the seat tilt and before or while curving.