An inflator according to this invention has a structure that an interior part of a housing is charged with a pressurized gas and a gas generating agent capable of generating a gas when combusted, wherein the pressurized gas formed only of an inert gas; a quantity of heat to be generated when the gas generating agent is combusted is set within a range of 6000 to 10000 J/g; and a molar ratio between the pressurized gas and the combustion gas generated by the combustion of the gas generating agent is set within a range of 20 to 40.