An airbag apparatus comprising an airbag that expands between a vehicle occupant and a side wall of a vehicle body. The airbag is divided into a plurality of expanding chambers that are connected by a plurality of gas flow channels. Among the expanding chambers, the cross-sectional area of the gas flow channel leading to the expanding chamber that expands first is the largest, the cross-sectional area of the gas flow channel leading to the expanding chamber that expands last is the smallest, and the expanding chambers expand in stages.