The present invention provides an inflator including a rupturable plate closing between the ignition device chamber having a gas discharge port and the pressurized gas chamber filled with a pressurized gas; a partition wall provided with a rod axially separating an interior of an ignition device unit housing, corresponding to the ignition device chamber, into two chambers; a combustion chamber provided on the side of a first end of the ignition device unit housing and provided with an igniter, and a diffuser chamber having the gas discharge port and provided on the side of a second end, which is an opposite end to the first end, the rupturable plate being circular and having, a circumferential edge fixing portion and a circular non-fixing portion on an inner portion other than the circumferential edge fixing portion, a fragile portion formed by not less than three grooves provided on a first surface facing the bottle, the grooves extending from a circumference of the circular non-fixing portion or from positions in the vicinity thereof, as a start point, toward a center of the circular non-fixing portion such that each of end points of the grooves does not reach the center of the circular non-fixing portion, the rod fixed to the partition wall and extending toward the rupturable plate, and the rod colliding with the rupturable plate due to a gas pressure generated by activation of the igniter, coming into contact with all of the end points in the fragile portion and rupturing the rupturable plate.