An airbag includes: a bag body forming the outer peripheral wall; and a regulating member disposed on an outer peripheral side of the bag body at a time of inflation completion of the bag body. The regulating member is configured so as to be able to regulate the bag body so that an area ranging from the vicinity of a joint portion for mutually joining the outer peripheral edge of the vehicle body side panel and the outer peripheral edge of the occupant side panel to a side of the gas inflow port at the time of inflation completion of the airbag is made smaller in external dimensions and reduced in size than an area at a time of free inflation state of the bag body.