An airbag for storage at an inner side of the backrest in a vehicle width direction includes a lower folded portion made by folding a lower area of the airbag that is to be situated at a lower side of vehicle upon inflation and deployment, an upper folded portion made by folding an upper area of the airbag that is to be situated at an upper side of vehicle upon the inflation and deployment, and a middle folded portion made by folding a middle area of the airbag that is to be situated between the lower area and the upper area upon the inflation and deployment, wherein the middle folded portion is a roll-folded portion such that a face of the airbag facing away from the inner side upon the inflation and deployment faces inward in the roll-folded portion, the roll-folded portion having the upper folded portion wrapped therein.