An airbag (11) by which, when a vehicle (20) including two seats arranged in a vehicle width direction is subjected to a side collision, an occupant (P10, P11) sitting in a seat (22L, 22R) at the opposite side of the collision is restricted from moving to the collision side. The airbag (11) has, in the maximally-expanded status, an outline shape seen from the front side of the vehicle. This outline shape has: an airbag upper part (11U) that forms a part equal to or higher than a position corresponding to a shoulder of a seat back (25L, 23R) and that protects the neck and head for example of the occupant (P10, P11); and an airbag lower part (11D) that forms a part equal to or lower than a position corresponding to a shoulder of the seat back (25L, 23R) and that protects the shoulder and arm for example of the occupant (P10, P11). The airbag (11) is configured to have an entire shape that has the inner outline of the occupant (P10, P11) in the vehicle width direction.