An airbag apparatus includes a cylindrical-shaped inflator that generates gas, an airbag that is inflated by the gas generated by the inflator, a retainer body to which the inflator is attached and fixed and including an insertion hole into which the inflator can be inserted, a flange member that is caulked or press-fitted to the inflator and that constrains the inflator from moving toward one end side in an axial direction and includes an insertion hole into which the inflator can be inserted and a flange surface extending outward in a radial direction, and a retaining member that constrains the inflator from moving toward another end side in the axial direction. The inflator is attached and fixed to the retainer body by bolting the retainer body and the flange member.