An airbag includes a front panel and a rear panel, and the outer peripheries of the front panel and the rear panel are joined together. Each of the front panel and the rear panel has a propeller shape in which a plurality of blade portions extend radially from a center portion such that recessed portions are formed between adjacent blade portions in plan view before the front panel and the rear panel are joined. The plurality of blade portions of the front panel are joined to corresponding recessed portions of the rear panel, and the plurality of blade portions of the rear panel are joined to corresponding recessed portions of the front panel.