In a gas generator of the invention, a closure member is disposed so as to be connected to an outer circumferential wall side along a circumferential edge thereof in a housing and to close an outlet port which is formed in a ceiling wall of the housing. Thin breakable portions adapted to be broken when a squib is activated are formed in the closure member so as to extend substantially radially from a center of the closure member in a rectilinear fashion. When the squib 19 is activated, the breakable portions are broken, whereby portions of the closure member 11 which lie between the breakable portions are individually made to be opened radially. A stopper portion is formed on a closure member side surface of the ceiling wall such that the opened door portions are brought into abutment therewith so that the opening angles of the door portions become less than 90.degree. while ensuring an angle at which combustion gas is allowed to flow out.