An airbag apparatus which has an airbag provided at least either on the left side or right side of the circumferential wall with an exhaust opening is disclosed. The exhaust opening is comprised of part of a lap portion of the airbag where a terminal of a front section of the circumferential wall and a terminal of a rear section of the circumferential wall overlap in such a manner that the front section is located inside of the rear section. The front section includes a band portion that extends toward the projecting point of the airbag at full inflation and is coupled to the rear section. The band portion is folded away from the terminal of the front section in a folded-up configuration of the airbag.