A gas generator for inflating an air bag, comprising: a pressurized gas chamber having an outer shell formed by a cylindrical pressurized gas chamber housing, the pressurized gas chamber being filled with pressurized gas; a combustion chamber connected to a first end portion of the pressurized gas chamber and including an ignition device and a gas generating agent accommodated in a combustion chamber housing; a diffuser portion connected to a second end portion of the pressurized gas chamber and provided with a gas discharge port; a first rupturable plate blocking a first communication passage between the pressurized gas chamber and the combustion chamber; a second rupturable plate blocking a second communication passage between the pressurized gas chamber and the diffuser portion; and a filter disposed and positioned in the pressurized gas chamber to cover the second rupturable plate, the filter allowing, during actuation, the combustion gas from the combustion chamber and the pressurized gas to pass therethrough and be discharged therefrom in response to rupturing the second rupturable plate, characterized in that the filter is made of a laminate of a wire mesh, a metal wire, or a compression-molded body of the laminate, and the diffuser portion has a filter attaching portion including either one of an annular protruding portion and a plurality of independent protruding portions which protrude from a portion thereof facing the pressurized gas chamber, the filter has either one of an annular groove and a plurality of concave portions on a peripheral surface thereof, and the filter is attached by the filter attaching portion fitted to one of the annular groove and the plurality of concave portions on the peripheral surface of the filter. a pressurized gas chamber having an outer shell formed by a cylindrical pressurized gas chamber housing, the pressurized gas chamber being filled with pressurized gas; a combustion chamber connected to a first end portion of the pressurized gas chamber and including an ignition device and a gas generating agent accommodated in a combustion chamber housing; a diffuser portion connected to a second end portion of the pressurized gas chamber and provided with a gas discharge port; a first rupturable plate blocking a first communication passage between the pressurized gas chamber and the combustion chamber; a second rupturable plate blocking a second communication passage between the pressurized gas chamber and the diffuser portion; and a filter disposed and positioned in the pressurized gas chamber to cover the second rupturable plate, the filter allowing, during actuation, the combustion gas from the combustion chamber and the pressurized gas to pass therethrough and be discharged therefrom in response to rupturing the second rupturable plate, characterized in that the filter is made of a laminate of a wire mesh, a metal wire, or a compression-molded body of the laminate, and the diffuser portion has a filter attaching portion including either one of an annular protruding portion and a plurality of independent protruding portions which protrude from a portion thereof facing the pressurized gas chamber, the filter has either one of an annular groove and a plurality of concave portions on a peripheral surface thereof, and the filter is attached by the filter attaching portion fitted to one of the annular groove and the plurality of concave portions on the peripheral surface of the filter.