A gas generator includes: a housing having closure and diffuser shells; an inner tube member accommodating an igniter, an opening end of the inner tube member being located opposing a bottom plate of the closure shell and defining a ventilating portion formed at the opening end, the inner tube member being fixed to an igniter attaching portion, and having a cylindrical portion projecting inwardly in the closure shell, and a passage being formed between the inner tube member and the igniter attaching portion to reach the ventilating portion; an outer circumference cylindrical member provided in the combustion chamber and one end opening thereof being located at the bottom plate; and an annular porous member defining a space above the combustion chamber, the space being communicated to a gas discharge path defined between the outer circumference cylindrical member and a side surface of the diffuser shell through a communicating portion.