A side airbag apparatus includes a flat airbag formed of a pair of base fabric sheets, which are stacked in the widthwise direction of a vehicle. The airbag is stored in an outer side portion of a seat back portion of a vehicle seat. Inflation gas is supplied to the airbag in response to an impact from the exterior. This inflates and deploys the airbag between a body side portion of the vehicle and the vehicle seat. A pair of tucked portions are located rearward of a center of the airbag. The tucked portions are formed by folding parts of the airbag. The tucked portions are joined to each other while being vertically stacked. Further, the width of the airbag along the widthwise direction of the vehicle in the vicinity of the tucked portions when the airbag is inflated is larger than the width of a rear portion of an airbag having no tucked portion along the widthwise direction of the vehicle when the airbag is inflated.