A gas generator, comprising: a diffuser shell having a top plate and an upper circumferential wall plate provided with a gas discharge port, a closure shell having a bottom plate and a lower circumferential wall plate, a housing forming an outer shell and being obtained by joining and integrating the upper circumferential wall plate and the lower circumferential wall plate, inside the housing, an ignition device chamber defined by a cup-shaped container having an opening and a bottom surface, the cup shaped container being positioned at a central part of the bottom plate of the closure shell, and accommodating an ignition device therein, the opening of the cup-shaped container being fixed to the bottom plate of the closure shell and the bottom surface of the cup-shaped container not being fixed to the top plate of the diffuser shell, a combustion chamber formed to enclose the ignition device chamber and being loaded with a gas generating agent, an annular or flat plate-shaped filter (70; 170) disposed to face and be in contact with the top plate, a retainer supporting the filter (70; 170), the retainer including a bottom surface and a cylindrical wall surface extending from an outer circumferential edge of the bottom surface at least towards the top plate of the diffuser shell, the cylindrical wall surface having a combustion gas passage, the retainer supporting at least part of an outer circumferential surface (71; 171) and a lower surface (72; 172) of the filter (70; 170) by the cylindrical wall surface and the bottom surface, respectively, the retainer being fixed by directly or indirectly press-fitting the cylindrical wall surface to a circumferential wall plate which is formed by the upper circumferential wall plate and the lower circumferential wall plate, wherein the gas generator is so configured that during actuation the housing is deformed in both directions such that the top plate and the bottom plate deform away from each other, the retainer moving into contact with the top plate of the diffuser shell, the bottom surface of the retainer being deformed under the pressure in the combustion chamber while pressing the lower surface of the filter (70; 170), combustion gas passing through the filter (70; 170), then passing through the combustion gas passage in the cylindrical wall surface of the retainer and being discharged from the gas discharge port. a diffuser shell having a top plate and an upper circumferential wall plate provided with a gas discharge port, a closure shell having a bottom plate and a lower circumferential wall plate, a housing forming an outer shell and being obtained by joining and integrating the upper circumferential wall plate and the lower circumferential wall plate, inside the housing, an ignition device chamber defined by a cup-shaped container having an opening and a bottom surface, the cup shaped container being positioned at a central part of the bottom plate of the closure shell, and accommodating an ignition device therein, the opening of the cup-shaped container being fixed to the bottom plate of the closure shell and the bottom surface of the cup-shaped container not being fixed to the top plate of the diffuser shell, a combustion chamber formed to enclose the ignition device chamber and being loaded with a gas generating agent, an annular or flat plate-shaped filter (70; 170) disposed to face and be in contact with the top plate, a retainer supporting the filter (70; 170), the retainer including a bottom surface and a cylindrical wall surface extending from an outer circumferential edge of the bottom surface at least towards the top plate of the diffuser shell, the cylindrical wall surface having a combustion gas passage, the retainer supporting at least part of an outer circumferential surface (71; 171) and a lower surface (72; 172) of the filter (70; 170) by the cylindrical wall surface and the bottom surface, respectively, the retainer being fixed by directly or indirectly press-fitting the cylindrical wall surface to a circumferential wall plate which is formed by the upper circumferential wall plate and the lower circumferential wall plate, wherein the gas generator is so configured that during actuation the housing is deformed in both directions such that the top plate and the bottom plate deform away from each other, the retainer moving into contact with the top plate of the diffuser shell, the bottom surface of the retainer being deformed under the pressure in the combustion chamber while pressing the lower surface of the filter (70; 170), combustion gas passing through the filter (70; 170), then passing through the combustion gas passage in the cylindrical wall surface of the retainer and being discharged from the gas discharge port.