In an airbag device of the invention, by making use of a retainer which is made up of a retaining part which is provided on an outside of an inflator and bolt parts, an airbag and the inflator are fixed to a fixing wall portion of a case by fastening nuts onto the bolt parts which are caused to protrude from the fixing wall portion of the case in such a state the retaining part is housed within the airbag. A support projection is formed on the fixing wall portion in such a manner as to protrude therefrom toward the inflator side. When the nuts are fastened, the retainer moves the retaining part toward the fixing wall portion side, so that the inflator is held between and retained by abutment portions formed on the retaining part and the support projection.