According to the air bag apparatus, for example, a side air bag apparatus 20, in a mode in which a portion on a side of a connector portion 33 of an inflator 30 is exposed from a cylindrical portion 51 of a retainer 50, the inflator 30 is contained in the retainer 50. Further, an exposed portion of the inflator 30, that is, a portion thereof exposed from the cylindrical portion 51 of the retainer 50 is fastened together with an extended portion 52 substantially in a shape of a half cylinder extended from the cylindrical portion 51 of the retainer 50 by a clamp mechanism 60 in a state of being covered by an extraneous portion of a base cloth extended from an opening portion 41 of the air bag 40.