An airbag device for a passenger's seat includes an airbag, cylinder type inflator, a casing, and a retainer. The airbag is provided with an opening for inflowing an expansion gas therethrough and is capable of being folded. The inflator is capable of producing and supplying the expansion gas to the opening of the airbag. The casing may receive the folded airbag and the inflator therein. The retainer is adapted to engage a peripheral edge of the opening of the airbag and is mounted to the casing with the peripheral edge of the airbag being disposed and compressed between the retainer and the casing. The retainer includes presser portions, which abut the inflator when mounted to the casing. The casing includes supports, which abut the inflator. The inflator is disposed between the presser portions of the retainer and the supports of the casing, thereby securing the inflator within the casing.