An air bag apparatus having a constitution of developing an air bag folded to contain in a case to a rear side of a vehicle by making an expanding gas flow from a gas flow inlet in expanding to develop the air bag. The air bag is arranged with a through hole. The air bag is constituted by a ring-like member arranged with a lower expanded portion arranged with a gas flow inlet, an upper expanded portion arranged to an upper side of the lower expanded portion to communicate with a front side of the lower expanded portion, and a rear expanded portion arranged to connect to a rear part of the lower expanded portion and a rear part of the upper expanded portion at a surrounding of the through hole. The air bag is constituted to finish to expand by closing the through hole.