An airbag comprises an inside base cloth, an outside base cloth, and a regulating member (tether) provided between these. The regulating member is respectively joined to front and rear portions of the inside base cloth at front-side and rear-side joint portions. The longitudinal length between the front-side and rear-side joint portions of the regulating member is shorter than that between the front-side and rear-side joint portions of the inside base cloth. The rear-side joint portion is positioned on a forward side of a front end portion of a side frame (a first side frame) but on a rearward side of an imaginary line connecting a hip point and a center position of a shoulder portion of a human body dummy for side-collision test (AM50) seated in a vehicle seat in a side view. A back-portion protection area of the airbag is constituted by a part of the inside base cloth.