A gas generator comprising: a diffuser shell which has a top plate and an upper circumferential wall plate provided with a gas discharge port, a closure shell which has a bottom plate and a lower circumferential wall plate, a housing, as an outer shell, obtained by joining and integrating only the upper circumferential wall plate and the lower circumferential wall plate, in the housing, an ignition device chamber defined by a cup-shaped container which accommodates an ignition device, a combustion chamber defined to surround the ignition device chamber and charged with a gas generating agent, an annular or flat-plate filter disposed to face the top plate and to be spaced from the upper circumferential wall plate, a first support member abutting against and supporting an upper surface of the filter, a second support member abutting against and supporting a lower surface of the filter, the first support member including, a disk-shaped portion and an annular wall portion, one surface of the disk-shaped portion abutting against the top plate (11 a), the other surface of the disk-shaped portion abutting against the upper surface of the filter, the annular wall portion abutting against a part of an outer circumferential surface of the filter; characterized in that the disk-shaped portion is provided with a plurality of communication holes formed only at a section in contact with the upper surface of the filter, the second support member includes, a substantially cylindrical wall portion and an annular flat portion formed at a circumference in one end of the substantially cylindrical wall portion, and the substantially cylindrical wall portion is fixed to the housing, the annular flat portion abuts against the lower surface of the filter, and, during actuation, a gap is formed between the top plate (11 a) and the first support member and a gap is formed between the upper circumferential wall plate and the first support member, and when a pressure inside the combustion chamber increases due to combustion gas generated by combustion of the gas generating agent, the top plate of the diffuser shell is deformed. a diffuser shell which has a top plate and an upper circumferential wall plate provided with a gas discharge port, a closure shell which has a bottom plate and a lower circumferential wall plate, a housing, as an outer shell, obtained by joining and integrating only the upper circumferential wall plate and the lower circumferential wall plate, in the housing, an ignition device chamber defined by a cup-shaped container which accommodates an ignition device, a combustion chamber defined to surround the ignition device chamber and charged with a gas generating agent, an annular or flat-plate filter disposed to face the top plate and to be spaced from the upper circumferential wall plate, a first support member abutting against and supporting an upper surface of the filter, a second support member abutting against and supporting a lower surface of the filter, the first support member including, a disk-shaped portion and an annular wall portion, one surface of the disk-shaped portion abutting against the top plate (11 a), the other surface of the disk-shaped portion abutting against the upper surface of the filter, the annular wall portion abutting against a part of an outer circumferential surface of the filter; characterized in that the disk-shaped portion is provided with a plurality of communication holes formed only at a section in contact with the upper surface of the filter, the second support member includes, a substantially cylindrical wall portion and an annular flat portion formed at a circumference in one end of the substantially cylindrical wall portion, and the substantially cylindrical wall portion is fixed to the housing, the annular flat portion abuts against the lower surface of the filter, and, during actuation, a gap is formed between the top plate (11 a) and the first support member and a gap is formed between the upper circumferential wall plate and the first support member, and when a pressure inside the combustion chamber increases due to combustion gas generated by combustion of the gas generating agent, the top plate of the diffuser shell is deformed.