In the air bag according to the present invention, pouches 1g are furnished at least along a part of an opening 1d to introduce reaction gas from an inflator, and band-like ring member 1e' mounted on car body and having the plane in the direction approximately perpendicular to the cross-section of the opening 1d is inserted into the pouches 1g.Therefore, the plane of the band-like ring member 1e' runs approximately in parallel to the direction of the introduced reaction gas, and the band-like ring member 1e' is not enlarged in the direction perpendicular to the direction of the introduced reaction gas. As the result, the module can be designed in smaller size. Also, by furnishing the pouches 1g on the periphery of the opening and by inserting the band-like ring member 1e' into the pouches 1g, the number of the base cloths of the air bag can be reduced by one cloth.