A gas generator includes, a housing integrated at an upper circumferential wall plate and a lower circumferential wall plate, an ignition device chamber defined by a cup-shaped container disposed at a bottom plate of the housing, a combustion chamber surrounding the ignition device chamber, an annular filter facing a top plate of the housing, and a retainer supporting the filter and including an annular bottom surface supporting a lower surface of the filter and a cylindrical wall surface being press-fitted directly or indirectly to a circumferential wall plate of the housing. A circumferential wall of the cup-shaped container in the vicinity of the bottom surface abuts against or is positioned close to an inner circumferential surface of the filter. During activation, when the housing deforms, the annular filter gas-tightly attaches to the top plate of the diffuser shell, and a combustion gas is discharged radially outward through the filter.