In a side impact air bag system, a bag (7) is formed of a main body (9) opposite the passenger's abdomen and an upper inflation unit (11) opposite the passenger's chest. A tip portion of the upper inflation unit in the longitudinal direction is deleted and the connection of a gas intake portion (14) to an inflator (5) is shortened to reduce the length of the upper inflation unit as compared to that of the main body. This renders the length of the upper inflation unit (11) shorter than the main body (9) when the bag (7) is inflated, to relax the shocks on the passenger's chest when the bag is inflated. It permits positively inflating the bag in the narrow space between the vehicle body and the passenger's seat without excessively increasing shocks to the passenger's chest. <IMAGE>
