The present invention provides an inflator reduced in size and improved in rupturability of a rupturable plate. A rupturable plate 29 is fixed between a gas introducing chamber 22 and an igniter accommodating chamber 26, and the central axes of an igniter 24 and the rupturable plate 29 are coincident with each other. For this reason, rupturability of the rupturable plate can be improved at the time of actuation.