The present invention provides a gas generator, including in a cylindrical housing, an ignition device chamber provided with an ignition device, a gas inflow chamber having a gas discharge port and a pressurized gas chamber being arranged in the above order from the first end, a rupturable plate closing between the pressurized gas chamber and the gas inflow chamber and being fixed to an annular fixing portion, a breaking device for the rupturable plate being arranged between the ignition device chamber and the gas inflow chamber, the breaking device including a base, a rod main body which extends from the base towards the rupturable plate, and an enlarged-diameter portion at a distal end of the rod main body, and during actuation, the base moving in the axial direction, the enlarged-diameter portion being positioned inside the pressurized gas chamber after punching out the rupturable plate, and the rod main body portion being positioned so as to oppose the annular fixing portion in a radial direction, a pressurized gas passing through a gas passage and then is discharged from the gas discharge port, the gas passage including a first annular passage in which a width between the enlarged-diameter portion and the annular fixing portion varies and a second annular passage between the rod main body portion and the annular fixing portion, a cross-sectional area of the second annular passage being the smallest among the cross-sectional area of the second annular passage, an area of a minimum width portion of the first annular passage and a total opening area of the gas discharge port, and Q