Occupant protection system

In order to provide an occupant protection system in which an airbag module can easily be mounted to a seat, a retainer (16) is placed on the inner surface of the lower part of an airbag (12). Stud bolts (20) projecting downward from the lower surface of the retainer (16) are passed through bolt insertion holes (12a) of the airbag (12), to which first nuts (24) are tightened from the exterior of the airbag 12 through a patch (22); thus, the airbag (12) is joined to the retainer (16). A gas generator (14) is joined to the retainer (16) with a bracket (30). A seat pan (8) has a recess (28) for accommodating the patches (22) and the first nuts (24) when the retainer (16) to which the airbag (12) and the gas generator (14) have been joined in advance is mounted thereto.