An inflation portion of an airbag is divided by a partitioning member into an upstream inflation portion and a downstream inflation portion. Part of the partitioning member forms a pair of overlapping portions overlapped with each other in a band-like shape inside the upstream inflation portion. The overlapping portions are joined to each other by a joint portion located at a boundary between the overlapping portions and non-overlapping portions. The partitioning member includes an opening, which is formed by cancelling the joint between the overlapping portions by the joint portion, and a pressure regulating valve, which includes a pair of valve bodies, which sandwiches the opening. Stress relaxation portions are provided at the overlapping portions. The stress relaxation portions relax stress acting on parts of the joint portion that are adjacent to the opening as the pressure regulating valve is opened.