To provide an air bag system capable of stably reproducing the required spreading shape and occupant protecting performance at the time of being inflated irrespective of eventual error of the mounting position on the vehicle, the dimensional accuracy of the air bag itself, etc.

A shape (inflating dimension adjusting part 21a) capable of adjusting autonomously the amount of contraction in the longitudinal direction of a lower edge 15 caused by inflation of an inflator when it is actuated is formed as an inflating dimension adjusting mechanism at least on one side of the bottom 11a of a cylindrical inflation part 11 of this curtain-shaped air bag 3 and a front pillar mounting point 32a or a rear pillar mounting point 34a. Thereby, the distance between the end mounting points 32a and 34a arranged in the longitudinal direction of the lower edge member 18 can be adjusted by the gas pressure. Even in the case there is error in the positions of the mounting points 32a and 34a on the vehicle side, the dimensions of the air bag itself, etc., the error is absorbed or relieved and the air bag 3 can be spread in the required position with a high reproducibility.