The driver tilts his/her head to the direction of the curve center while the head movement of the passenger is opposite to the driver. Moreover, it is known that the driver does not get carsickness comparing with the passenger. Thus, we hypothesize that driver's head-tilt strategy has the effect to reduce motion sickness. So far, we proposed a mathematical model of the motion sickness incidence caused by the head movement in 3D space based on subjective vertical conflict theory. In this paper, we, thus, investigate motion sickness incidence of the head movements measured in the real car experiments using the proposed model. The results demonstrate that the head tilts against centrifugal acceleration have the effect to reduce motion sickness. Based on the results, we propose a novel posture control device for decreasing the passenger's head roll in curve driving. It is shown that the proposed device potentially has the effect to decrease the passenger's head roll angle, which is expected to reduce the motion sickness.