A gas generator, includes: a housing having closure shell, a diffuser shell including a gas discharge ports, and a flange portion; an inner tube member defining a combustion chamber in an outside thereof for accommodating a gas generating agent, the igniter being attached to the closure shell such that the igniter projects inside from a hole formed in the bottom plate of the closure shell, the annular porous member having a plurality of holes which are smaller than the size of the gas generator, defining a space above the combustion chamber, a total opening area (A1) of the gas discharge ports being smaller than a total opening area (A2) of a gas pathway extending from the combustion chamber to the gas discharge ports (A1