An airbag includes an insert opening. An inflator is inserted into the airbag via the insert opening and set in a holding section of a retainer which is disposed inside the airbag. The retainer includes, at its end region disposed towards the insert opening of the airbag, two retaining pawls that are in engagement with a periphery of the insert opening. The insert opening is composed of a plurality of slits formed on a base material of the airbag such that one or more area among the slits serve as a bent sealing region that is bent inward and covers an outer periphery of the inflator. The slits include a pair of retaining slits that intersect each other. Each of the retaining pawls protrudes from a terminal of each of the retaining slits. A peripheral area of the retaining slits constitutes the bent sealing region.