A gas generator composition reduced in the concentration of harmful gases, particularly carbon monoxide and nitrogen oxides, in the gas generated therefrom to such a level as to put the composition to practical use as an automotive air bag system. The composition comprises a nitrogenous organic compound such as an organic compound having an amino or amido group or a tetrazole derivative and an oxygenous inorganic oxidizing agent such as KNO3, Sr(NO3)2 or KC1O4 as the essential components and contains manganese dioxide having a specific surface area of 50 m /g or above, copper oxide having a specific surface area of 1 m /g or above, a molybdenum oxide catalyst comprising at least one member selected from the group consisting of molybdenum dioxide, molybdenum trioxide, molybdic acid and ammonium molybdate, or a mixed catalyst comprising manganese dioxide and one or more metal oxides selected from the group consisting of copper oxides, cobalt oxides, iron oxides and silver oxides.