A side airbag having a high pressure chamber and a low pressure chamber divided by a sewn portion, a diffuser that distributes gas for inflation to the high pressure chamber and the low pressure chamber, and a check valve that regulates flowing of gas from a high pressure chamber side to a low pressure chamber side. A partitioning wall, that divides a lower chamber (high pressure chamber) and an upper chamber (low pressure chamber) within a side airbag, is structured by a portion of a lower base cloth (high pressure chamber base cloth) that structures the lower chamber or a portion of an upper base cloth (low pressure chamber base cloth) that structures the upper chamber. A check valve, that permits flowing of gas, jetted-out from a gas jetting-out portion of an inflator, from the upper chamber to the lower chamber and that regulates flowing of gas in an opposite direction, is provided integrally with the partitioning wall.