A gas generator includes, a housing having a circumferential wall portion defining a gas discharge port, a first end, and a second end, the first and second ends being closed, a cup-shaped inner tube member disposed in the housing, an inside thereof accommodating an ignition device, an outside thereof being a combustion chamber charged with a gas generating agent, a bottom surface of the inner tube member being located in a bottom plate of the first end of the housing, an opening end portion of the inner tube member being provided with a ventilating portion for allowing a combustion product to pass therethrough, an igniter attached to an igniter attaching portion formed at the second end of the housing, and a passage for the combustion product formed between an inner wall surface of the inner tube member and the igniter attaching portion, and reaching the ventilating portion.