A gas generator has an inner cylindrical member disposed in a cylindrical housing, an annular first combustion chamber provided radially in the outside of the inner cylindrical member, and, in the combustion chamber, a ratio of its radial average distance to its axial average length is adjusted to be in the range of 0.2 to 2.0. The gas generator has a retainer which changes a flow of a gas ejected from a through-hole in the inner cylindrical member and has a cylindrical portion for collecting fluid and semifluid combustion products included in the gas.