An airbag includes a tuck that is formed by tucking a part of the outer circumferential wall toward an interior of the airbag. The tuck includes two opposing walls, a turn-round region that forms a bottom of the tuck and outermost ends of the opposing walls adjacent to and continuing to an outer surface of the airbag. The airbag further includes a temporary joint that joins the opposing walls of the tuck together in such a manner as to disjoint when an inner pressure of the airbag at inflation is increased. The temporary joint is formed into such a line that extends toward the turn-round region and has a first end proximate the outermost ends and a second end in the vicinity of the turn-round region.