An airbag includes a plurality of base materials made of a sheet body. The base materials include: an occupant side panel; a vehicle body side panel; a front side panel; and a rear side panel. Corresponding edge portions of the occupant side panel, the vehicle body side panel, the front side panel and the rear side panel are configured to be joined to each other by a planar joining operation. Outer peripheral edges of the front side panel and the rear side panel are joined to corresponding edge portions of the vehicle body side panel and the occupant side panel, respectively. The front side panel and the rear side panel are separated from each other on a front-rear direction side, when the inflation is completed, by joining corresponding left and right edges of the vehicle body side panel and the occupant side panel to each other.