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 inflator, the aforementioned air bag being folded, and a diffuser mounted about the inflator. The air bag is placed in a side-by-side relation to the inflator and located behind the inflator. The diffuser is provided with numerous holes for supplying the gas. The diffuser acts to regulate flow of the gas from the inflator.