An airbag that includes an inner bag having a facing surface configured to face a passenger of a vehicle in the deployed state, an outer bag that surrounds the outer periphery of the inner bag with the facing surface exposed, and two protrusion parts which are provided in the outer bag and are respectively disposed on the left and right sides in the width direction of the vehicle with respect to the facing surface so as to protrude rearward. The inner bag and the outer bag may be partitioned by a common partition wall, with a ventilation port formed in the partition wall. Moreover, the inner bag and the outer bag may each be independent bag bodies, a ventilation port may be formed in both the inner bag and the outer bag, and the inner bag and the outer bag may be coupled around the ventilation port.