An air bag system for protecting a passenger seated on the front passenger seat of a vehicle. In an initial stage, the air bag is inflated mildly. In the subsequent stage, the bag is inflated also mildly. The directivity of produced gas can be reduced. The air bag can be inflated mildly while maintaining a large area. The air bag system comprises an air bag case having an opening on its top side, an inflator disposed inside the air bag case, the aforementioned air bag folded in the air bag case, and a diffuser mounted between the inflator and the opening in the case. The air bag is placed in a side-by-side relation to the inflator and located behind the inflator inside the case. The diffuser is provided with numerous holes for supplying the gas. The diffuser acts to regulate flow of the gas from the inflator. At least when the gas is produced from the inflator, a space is created between the outer surface of the inflator and the diffuser.