corresponding to the arm rest 36 further the inner liner 34 may be reduced in thickness as compared with a normal door trim to facilitate the expansion of the airbag body 30 furthermore an easy to rupture portion may be provided so that opening or rupture easily occurs in a part of the inner liner 34 a plurality of easy to rupture portions may be provided when it is desired to attach the arm rest 36 to the side door 32 the arm rest 36 is placed in a predetermined position on the inner liner 34 of the side door 32 bolts 38 are successively penetrated into the arm rest 36 the inner liner 34 and the airbag body 30 so as to be fastened to the innerpanel 33 of the side door 32 by weld nuts 40 or the like other parts can be attached to the side door 32 in the same manner as described above the airbag body 30 is mounted to the door innerpanel 33 by the inner liner 34 so as to be interposed between the door innerpanel 33 and the inner liner 34 however a part of the airbag body 30 may be hooked and secured to the side door 32 with clips or the like injection nozzles of an inflator 42 are securely fitted to the inflator mounting holes 30d used for the introduction of gas into the airbag body 30 in advance the inflator 42 is securely fitted in a concave portion or recess 33a defined in the door innerpanel 33 since the inflator 42 is reduced.