An inflator attachment structure for attaching an inflator of an airbag apparatus (10; 100; 200) to a retaining member of the airbag apparatus (10; 100; 200), comprising: a first flange member (32; 110; 212) including a first cylindrical part (38; 216) having a shape matching a substantially cylindrical part provided on a lateral surface of the inflator, and including a first flange part (40; 112; 216) extending outwardly in a radial direction from the first cylindrical part (38; 216), the first flange member (32; 110; 212) being engaged with the inflator from one side of the part provided on the lateral surface of the inflator in an axial direction; and a second flange member (34; 210) including a second cylindrical part (44; 38) having a shape matching the part provided on the lateral surface of the inflator, and including a second flange part (46; 214) extending outwardly in a radial direction from the second cylindrical part (44; 38), the second flange member (34; 210) being engaged with the inflator from another side of the part provided on the lateral surface of the inflator in the axial direction, wherein the first flange member (32; 110; 212) includes a securing part (60; 114; 220) for use in bolt securement to the retaining member, characterized in that the substantially cylindrical part of the inflator is a substantially cylindrical projecting part provided on the lateral surface of the inflator, the first flange member (32; 110; 212) is pressed onto the inflator from one side of the projecting part in the axial direction to be engaged with the inflator, the second flange member (34; 210) is pressed onto the inflator from the other side of the projecting part in the axial direction to be engaged with the inflator, and the second flange member (34; 210) is nipped between the first flange member (32; 110; 212) and the retaining member when the first flange member (32; 110; 212) is bolt-secured to the retaining member. a first flange member (32; 110; 212) including a first cylindrical part (38; 216) having a shape matching a substantially cylindrical part provided on a lateral surface of the inflator, and including a first flange part (40; 112; 216) extending outwardly in a radial direction from the first cylindrical part (38; 216), the first flange member (32; 110; 212) being engaged with the inflator from one side of the part provided on the lateral surface of the inflator in an axial direction; and a second flange member (34; 210) including a second cylindrical part (44; 38) having a shape matching the part provided on the lateral surface of the inflator, and including a second flange part (46; 214) extending outwardly in a radial direction from the second cylindrical part (44; 38), the second flange member (34; 210) being engaged with the inflator from another side of the part provided on the lateral surface of the inflator in the axial direction, wherein the first flange member (32; 110; 212) includes a securing part (60; 114; 220) for use in bolt securement to the retaining member, characterized in that the substantially cylindrical part of the inflator is a substantially cylindrical projecting part provided on the lateral surface of the inflator, the first flange member (32; 110; 212) is pressed onto the inflator from one side of the projecting part in the axial direction to be engaged with the inflator, the second flange member (34; 210) is pressed onto the inflator from the other side of the projecting part in the axial direction to be engaged with the inflator, and the second flange member (34; 210) is nipped between the first flange member (32; 110; 212) and the retaining member when the first flange member (32; 110; 212) is bolt-secured to the retaining member.