A gas generator includes, a cylindrical housing having a port and closed at both ends, an ignition device attached to one end of the housing, a first chamber provided in the housing at a position close to the ignition device and filled with a first agent, a cylindrical filter provided in a space adjacent to the first chamber, a second chamber provided inside the filter and filled with a second agent, and a retainer disposed between the first and second chambers, the retainer having an annular flat surface and an annular circumferential wall surface that protrudes from an inner circumferential edge of the annular flat surface, the annular flat surface abutting one end of the filter and the annular circumferential wall surface abutting a portion of an inner circumferential surface of the filter, an inner diameter of the second chamber being smaller than that of the first chamber.