An inflatable member is restricted to expand outwardly in the vehicle width direction and allowed to expand upwardly to a position facing a side of an occupant's head when a lateral collision occurs. A belt guide mounted on the shoulder portion of a seat back guides a restraint webbing, including an inflatable member that is provided for supporting the head of a vehicle occupant when inflated, and allows a guide hole to be vertically enlarged by being projected upwardly in the vehicle vertical direction when the inflatable member is inflated. The inflatable member is thereby expanded so as to form a vertically extended shape regulated by the guide hole and disposed to an upward position facing the side of the occupant's head when a lateral collision occurs.