A gas generator includes: a pressurized gas chamber formed by a cylindrical pressurized gas chamber housing filled with a 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 therein, 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, 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.