An airbag includes a main inflation portion, in which an inflator is arranged, and an auxiliary inflation portion, the volume of which is smaller than that of the main inflation portion. The main inflation portion is deployed forward in a space between an occupant and a side wall of the vehicle. Prior to the deployment of the main inflation portion outside the seat back, the auxiliary inflation portion is inflated inside the seat back with inflation gas from the inflator to push the occupant toward the inner side. The main inflation portion and the auxiliary inflation portion each have a communication port that connects the main inflation portion and the auxiliary inflation portion to each other. The airbag includes an inflation promoting structure, which promotes the inflation of the auxiliary inflation portion compared to the inflation of the main inflation portion.