The Statistical Design Support System (SDSS) produces a new practical optimal design methodology with which optimization can be done using a small number of case studies. In prior study, the authors applied it to the optimization of design parameters of occupant restraint system, and have tried to reduce the injury criteria of occupants based on the crash simulation. However, response surfaces did not have enough accuracy in approximation. Therefore the authors made detailed investigation on the relationship between occupant restraint system and injury criteria. This paper also discusses the possibility to improve approximating accuracy of response surfaces for injury criteria. Furthermore, based upon the results of detailed effectivity study, only the important design factors were chosen for the optimum design analysis. In the optimization analysis, the occupant’s figure was handled as an uncertain factor.