A gas generating composition having a low combustion temperature and a low heat of combustion at the time of burning and therefore enabling downsizing of gas generators themselves, which comprises as the essential components nitroguanidine and an oxidizing agent comprising (a) nitrates or nitrites of alkali metals or alkaline earth metals, (b) oxides or multiple oxides of metals selected from among copper, cobalt, iron, manganese, nickel, zinc, molybdenum and bismuth, or a mixture of the components (a) and (b).