An airbag of an airbag apparatus includes a vent mechanism. The vent mechanism includes a vent hole, a belt member including a cover portion for covering the vent hole and two connecting sections extending from both sides of the cover portion and serving as tethers, an outer panel located on an outer side of the cover portion, an inner panel located on an inner side of the cover portion and provided with an inner opening. The vent hole is formed on the outer panel. The connecting sections are run through the guide section so as to move toward the vent hole and let the cover portion loosen and open the vent hole when an occupant side wall of the airbag catches an occupant and moves toward a vehicle body side wall. The guide section also holds the connecting sections such that the connecting sections serve as the tethers.