A gas generator includes an inflator and a retainer. The retainer includes a holding portion, which partially covers the inflator. The inflator has an attaching protrusion for determining the position relative to the retainer. The retainer includes an engagement base and a tongue piece. The tongue piece extends in an axial direction of the inflator and from a position separated from the engagement base in the circumferential direction of the holding portion. The engagement base and the tongue piece each have an engagement portion. The engagement portions face each other to form an engagement space therebetween, into which the attaching protrusion enters. The engagement space has an inlet the width of which is less than the size of the positioning protrusion. When the inflator causes the positioning protrusion to increase the width of the inlet, the tongue piece is elastically deformed about a basal end of the tongue piece.