Abstract: 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 closi