The present invention is an inflator with an improved reliability wherein a charge holder 37 for giving directivity to an detonation wave generated due to combustion of a priming 36 is provided to an igniter 26, thereby promoting rupture of a rupturable plate 19. A cross-shaped notch is applied to a surface of the rupturable plate 19, and when the rupturable plate 19 receives a rupture pressure from the igniter 26, four portions of the rupturable plate lap so that the outflow of the pressurized medium can be secured and fragments cannot be made. An inflator housing 12 and a diffuser 20 are connected by screwing a male screw portion on an outer surface of the end portion in the inflator housing 12 into a female screw portion on an inner surface of the end portion in the diffuser portion 20. An existing gas cylinder can be used in this method. A diffuser portion 320 is connected in the opening portion side of a housing 312, and a gas discharging port 340 is connected to the diffuser portion 320. The outflow direction of the pressurized medium and the axial directions of the inflator housing 312 and the gas discharging port 340 are coincident with each other at actuation. The diffuser portion 520 is connected in the opening portion side of the inflator housing 512, and the gas discharging port 540 is connected to the diffuser portion 520, and two filter means 400a and 400b are arranged. After actuation, foreign objects included in the pressurized medium can be removed by the filter means 400a and 400b.