A frame side plate part of a frame member of a seat along a side surface of a backrest; and a bracket attached to the frame side plate part in a condition where an airbag cushion and an inflator are secured. The frame side plate part has a notch region where a predetermined range of a front edge is cut out The bracket has: a base part which is fitted into the notch region of the frame side plate part and to which the airbag cushion and the inflator are secured; and a reaction force plate bent from a front side of the base part and extends to one side in a vehicle width direction. Guide grooves are formed on an upper edge and lower edge of the notch region, and the base part can be inserted into the notch region from the front along the guide grooves.