The present invention provides a gas generator, including a housing having therein an ignition device chamber, which is provided with a gas discharge port and accommodates an ignition device, and a pressurized gas chamber filled with a pressurized gas, the ignition device chamber and the pressurized gas chamber being separated by a partition wall having a communication hole, the partition wall being in a cup-like shape which includes a bottom surface and a circumferential wall surface, projects toward the pressurized gas chamber and has a flange portion at a portion corresponding to an opening in the cup-like shape of the partition wall, only the circumferential wall surface having the communication hole, the flange portion being fixed to the housing, a disk closing member being disposed inside the partition wall to be movable in the axial direction of the housing with a distance from the bottom surface, prior to activation, the communication hole being closed by a circumferential surface of the disk closing member, and at the time of activation, the closing member being moved toward the bottom surface by combustion products generated by the ignition device, thereby opening the communication hole and making communication between the ignition device chamber and the pressurized gas chamber.