To reduce cost and to improve productivity in a side airbag having a diffuser, which has a high-pressure chamber and a low-pressure chamber divided by a sewing part to distribute a gas for expansion to the high-pressure chamber and the low-pressure chamber, and a check valve, which regulates gas flow from the high-pressure chamber side to the low-pressure chamber side.

A lower chamber 28 (high-pressure chamber) and an upper chamber 32 (low-pressure chamber) in the side airbag 16 are divided by the sewing part S1 which sews an occupant side base cloth and an anti-occupant side base cloth in a lower base cloth 26 and an upper base cloth 30. The diffuser 22, which distributes the gas jetted out of a gas injection hole 18A of an inflator 18 to the lower chamber 28 and the upper chamber 32, is structured in a cylindrical shape protruding from the boundary part 38 of the upper chamber 32 and the lower chamber 28 toward the lower chamber 28 side by at least either one of a part of the lower base cloth 26 and a part of the upper base cloth 30.