An airbag has a main bag portion that is provided with an occupant protective surface which is disposed to face the occupant and an auxiliary bag portion that can be disposed on the left side of the storage part, between the main bag portion and a vehicle body-side member, when the inflation is completed. The auxiliary bag portion is configured to enable an inflating gas to flow into an inside thereof through the main bag portion, is configured to be inflated under control of a control mechanism in association with a collision mode of a vehicle, and is configured to be interposed between the main bag portion and the vehicle body-side member and to be capable of pushing a region of the occupant protective surface on a side of the auxiliary bag portion, which is inflated, toward a side of the occupant, when the inflation is completed.