A gas generator, comprising: a cylindrical ignition device chamber housing provided with a gas discharge port, accommodating an igniter and a gas generating agent therein, a cylindrical pressurized gas chamber housing filled with a pressurized gas, the cylindrical pressurized gas chamber housing being joined to the ignition device chamber housing to form an outer shell container; a closing member being provided with a fragile portion and closing between the ignition device chamber housing and the pressurized gas chamber housing; a multifunctional member that is movable in an axial direction during activation being arranged inside the ignition device chamber housing together with the igniter and the gas generating agent, the igniter closing an opening portion of the ignition device chamber housing and being fixed thereto; the multifunctional member having, a base plate portion and a cylindrical wall portion extending from one surface of the base plate portion, the cylindrical wall portion having an opening portion at a distal end thereof the base plate portion abutting against an inner wall surface of the ignition device chamber housing and having a first through-hole in a thickness direction, and the cylindrical wall portion having a smaller outer diameter than an outer diameter of the base plate portion, and being provided with a second through-hole in the thickness direction thereof, the opening portion abutting against the fragile portion of the closing member; a first plenum chamber enclosed by the base plate portion, the cylindrical wall portion and the closing member; a second plenum chamber formed by a cylindrical space between an outer wall surface of the cylindrical wall portion and the inner wall surface of the ignition device chamber housing, and the gas discharge port being provided in a circumferential wall portion of the ignition device chamber housing, to face the second plenum chamber; a gas generating agent charging chamber enclosed by the base plate portion, the igniter, and the inner wall surface of the ignition device chamber housing and charged with the gas generating agent, by pressure increase inside the ignition device chamber housing due to combustion gas generated by combustion of the gas generating agent, the multifunctional member being moved in the axial direction to rupture the closing member, the combustion gas passing through the first through-hole, the first plenum chamber, the second through-hole and the second plenum chamber, and then being discharged from the gas discharge port, the pressurized gas inside the pressurized gas chamber housing entering the first plenum chamber from an opening portion obtained by rupture of the closing member, passing through the second through-hole and the second plenum chamber, and then being discharged from the gas discharge port. a cylindrical ignition device chamber housing provided with a gas discharge port, accommodating an igniter and a gas generating agent therein, a cylindrical pressurized gas chamber housing filled with a pressurized gas, the cylindrical pressurized gas chamber housing being joined to the ignition device chamber housing to form an outer shell container; a closing member being provided with a fragile portion and closing between the ignition device chamber housing and the pressurized gas chamber housing; a multifunctional member that is movable in an axial direction during activation being arranged inside the ignition device chamber housing together with the igniter and the gas generating agent, the igniter closing an opening portion of the ignition device chamber housing and being fixed thereto; the multifunctional member having, a base plate portion and a cylindrical wall portion extending from one surface of the base plate portion, the cylindrical wall portion having an opening portion at a distal end thereof the base plate portion abutting against an inner wall surface of the ignition device chamber housing and having a first through-hole in a thickness direction, and the cylindrical wall portion having a smaller outer diameter than an outer diameter of the base plate portion, and being provided with a second through-hole in the thickness direction thereof, the opening portion abutting against the fragile portion of the closing member; a first plenum chamber enclosed by the base plate portion, the cylindrical wall portion and the closing member; a second plenum chamber formed by a cylindrical space between an outer wall surface of the cylindrical wall portion and the inner wall surface of the ignition device chamber housing, and the gas discharge port being provided in a circumferential wall portion of the ignition device chamber housing, to face the second plenum chamber; a gas generating agent charging chamber enclosed by the base plate portion, the igniter, and the inner wall surface of the ignition device chamber housing and charged with the gas generating agent, by pressure increase inside the ignition device chamber housing due to combustion gas generated by combustion of the gas generating agent, the multifunctional member being moved in the axial direction to rupture the closing member, the combustion gas passing through the first through-hole, the first plenum chamber, the second through-hole and the second plenum chamber, and then being discharged from the gas discharge port, the pressurized gas inside the pressurized gas chamber housing entering the first plenum chamber from an opening portion obtained by rupture of the closing member, passing through the second through-hole and the second plenum chamber, and then being discharged from the gas discharge port.