A holder for mounting an air bag module, the holder including a bottomed, substantially box-like body having a peripheral wall and an opening, the holder adapted to be fastened to a side plate of a seat back frame in a cavity of a back pad with the opening thereof facing a body of an automotive vehicle and with a bottom plate section thereof being applied to the side plate, and wrapped at the peripheral wall thereof by margin pieces which are provided around an opening portion in a side of a seat cover, rolled in the holder from the opening of the holder, and fastened to the holder, the air bag module having an air bag housed within an air bag case, the air bag module adapted to be incorporated in the holder and fastened to the side plate through the holder, whereby the air bag module is installed in the side of the seat back, the holder further including flanges, and at least first and second brittle sections serving as breaking portions, the peripheral wall including a front side wall section facing a front side of the seat back, a rear side wall section facing a rear side of the seat back, an upper side wall section, and a lower side wall section, the flanges projecting outwardly from edges of the front, rear, upper and lower side wall sections, at least a flange portion of the front side wall section having a thin metal plate embedded in an inner surface thereof, whereby the flange is reinforced by the metal plate, the first brittle section provided at a corner between the front side wall section and the upper wall section, and at a corner between the flange of the front side wall section and a flange of the upper wall section, and the second brittle section provided at a corner between the front side wall section and the lower wall section, and at a corner between the flange of the front side wall section and a flange of the lower wall section.