The number of fatalities in traffic accidents has been reduced continuously. One of the factors for such reduction may be improvement of safety devices. However, though total number of deaths has been reduced, many lives are still lost in traffic accidents. Nearly half of the deaths while driving automobile is in frontal crash. On the other hand, driver seating position is variable and the distance between the occupant and occupant restraint system becomes longer in rearmost position (RM) than mid position (MP) of seat slide, resulting in a delay of occupant restraint onset. Because of the delay of occupant restraint, pelvis restraint is also delayed and pelvis displacement increases. At that time, the motion of pelvis increases the tension of lap belt and it is transferred to the inboard shoulder belt through the thorough tongue. Tension in inboard shoulder belt increases the loading to the lower rib cage and may increase the risk of chest injury. This research examined the influence of seating in RM position to the occupant’s lower thorax injury and the influence on lower thorax injury by controlling pelvis behaviors in RM position. In this study, finite element (FE) simulations of the sled test in flat56km/h were conducted in MP and RM seat positions. Firstly, it was confirmed that the tension of the lap belt caused by pelvis motion transferred to the inboard shoulder belt and it compressed lower rib cage. Especially it seemed to occur in RM. Secondly, simulation was conducted by changing constraint conditions on pelvis translation and lateral axis rotation to confirm the effect on injury criteria in RM. Since the distance between the instrument panel and the occupant became longer in RM, knees were not constrained by instrument panel(IN-PNE), therefore chest deflection increased. It is confirmed that the lap belt tension was increased with the pelvis forward motion caused by reduction of restraint force, and the tension transferred to the shoulder belt, and consequently the deflection of the lower rib cage was increased. By constraining pelvis translation or rotation, or both of them, the constraint of the pelvis was improved and the chest deflection decreased in each condition. In case of fixing translation and rotation, there was an increase in acceleration of the pelvis and acceleration of T12 also increased through the lumbar spine. Therefore, chest deflection was reduced. In terms of the effect to the tension of seat-belt, there was not transferring of the tension to the shoulder belt from the lap belt. Since the tension of the shoulder belt decreased after 80ms, it was seen that loading to the chest from shoulder belt decreased. It was found that in order to reduce chest deflection in lower right side, it is effective not only reducing the load from inboard shoulder belt but also increasing a degree of constraint on the lumbar spine. Loading to the chest from the inboard shoulder belt was able to be reduced by suppressing pelvis rotation and it was effective to reduce chest deflection further.