An object of the invention is to provide an airbag that can be efficiently stored according to a storage space. The invention is a side airbag 1 stored in the backrest portion SA of a vehicle seat S, having a first airbag 3 which is inflated in a circular shape by gas supplied by an inflator 2 provided in the backrest portion SA when seen from the side of the backrest portion SA; a second airbag 4 provided so as to be located in the first airbag 3 in an inflated state; and a communicating path 5 that communicates the first airbag 3 and the second airbag 4 on the side of the backrest portion of the second airbag when seen from the side of the backrest portion, in which the second airbag 4 has a vertical width T2 set to be larger than the vertical width T3 of the communication path 5, and has at least the top portion 4b, the bottom portion 4c, and the front portion 4d separated from the first airbag 3 when seen from the side of the backrest portion SA.
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