An airbag is provided with a vent hole and a patch closing the vent hole. The airbag has a conically-shaped protrusion with the front narrowed when inflation is finished. The vent hole and patch are at the vehicle body side wall of the protrusion. The airbag has the reaction force-receiving contact portion that is partially recessed toward an occupant by receiving a reaction force substantially opposite to the approach direction of the occupant from the reaction force-applying support portion of the vehicle body structure when the occupant is cushioned, at the vehicle body side wall of the protrusion. The patch is positioned where the vent hole can keep closed when the airbag finishes inflating. The patch is not pressed against the reaction force-applying support portion but is spaced from the circumferential edge portion of the vent hole that moves when the reaction force-receiving contact portion is recessed.