Three apex portions of a triangular air bag 32 are folded toward a central portion of the air bag, respectively, so that the respective apex portions and a retainer are coupled together by a restricting member 39, and thereafter, the air bag 32 is folded up into an appropriate shape. In an earlier stage of deployment of the air bag 32, the inflation of the air bag 32 to its maximum capacity is restricted by the restricting member 39, whereby the air bag 32 is allowed to inflate quickly to a capacity which is smaller than its maximum capacity, thereby making it possible to get the air bag 32 ready for restraining an occupant. In a later stage of deployment of the air bag 32 that occurs thereafter, when an internal pressure of the air bag 32 reaches or exceeds a predetermined value, the restricting member 39 breaks or extends. |