A gas generator includes, a housing having a circumferential wall, a first end, and a second end, an inner tube member disposed in the housing, an outer space thereof being a combustion chamber, an outer circumference cylindrical member being such that one end opening abuts against a bottom plate of the second end, an annular porous member provided between the outer circumference cylindrical member and the cup-shaped inner tube member so as to form a space above the combustion chamber, a gas discharge path formed between the outer circumference cylindrical member and an inner wall surface of the circumferential wall portion of the housing; a communicating portion communicating the space and the gas discharge path, and being provided in the other end opening of the outer circumference cylindrical member; and a total cross-sectional area of the gas discharge port being smaller than a smallest cross-sectional area of a gas pathway.