Abstract: A gas generator includes a lower shell, an upper shell, a holding portion, an igniter, and a filter. The lower shell is made of a press-formed product formed by press-working one plate-shaped member made of metal, and includes a protruding cylindrical portion and an opening provided in the protruding cylindrical portion. The holding portion is formed from a resin-molded portion formed by attaching an insulating fluid resin material to a bottom plate portion so as to reach a part of an outer surface from a part of an inner surface of the bottom plate portion of the lower shell through the opening and solidifying the same, and includes an accommodation recess portion in a portion opposed to a top plate portion. The igniter is held by the holding portion while it is inserted in the accommodation recess portion from a side of the top plate portion and fitted therein.