An airbag device comprises a mounting plate and a retaining member, the mounting plate includes a housing portion which houses therein a part of an inflator, the retaining member includes a fixing portion fixed to the mounting plate at a location that is radially outward relative to the side surface of the inflator, a leg portion which has a first end portion provided continuously with the fixing portion, and extends from the first end portion toward the top surface of the inflator in the center axis direction, and an arm portion extending radially inward relative to the side surface from the second end portion of the leg portion, and the retaining member is in contact with a part of the inflator and is elastically deformed to retain the inflator in the housing portion, in a state in which the fixing portion is fixed to the mounting plate.