An airbag is formed by joining outer edges of an occupant-side wall and a vehicle-side wall each of which is made of a base cloth having flexibility. The occupant-side wall and vehicle-side wall are equal in circumferential length of the outer edge. The occupant-side wall includes a recessed region for receiving a head of a vehicle occupant and an annular raised region that surrounds the recessed region. At least one of the occupant-side wall and the vehicle-side wall includes at least one shortening region that reduces a circumferential length of an outer edge of the base cloth.