By nature, the driver's seat should be designed for work, while the passenger's seat should be built for comfort. This means that the functions of the seats are inherently different. Although many studies have been done on the driver's seat, the design and use of the passenger's seat have received little attention.This study examined a comfortable sitting posture in the passenger's seat. The results obtained have led to the development of two new devices.One device makes it possible for the seat cushion to move upward and forward as the seat tilts backward. The other device allows the upper portion of the seat back to tilt forward from the top of the lower seat back. These devices thus function to provide a comfortable sitting posture.This paper describes the new devices and presents the results of an investigation into a comfortable sitting posture for the occupant of the front passenger's seat. Factors discussed include the minimum cushion angle for preventing hip-sliding, allowable hip angle, knee angle and ankle angle for comfort, and related physiological observations.