An air bag system has an air bag, an inflator for inflating the air bag, the inflator having a first end and a second end with a flange portion, a casing for receiving the air bag and the inflator, the casing including an air bag receiving casing and an inflator receiving casing, and a cover portion for covering an opening portion of the casing. A pair of fitting holes are formed in two wall surfaces of the inflator receiving casing opposite to each other and the fitting holes are substantially equal in diameter to each other so that the inflator can be fitted into the fitting holes. The first end of the inflator is engaged with a first fitting hole through a spacer while the second end of the inflator is connected to a second fitting hole through the flange portion, and a position regulating member is provided in one of the casing and the spacer for preventing the inflator from shifting in position.