A gas generating composition for use in a pretensioner, which generates less CO gases and chlorine-containing gases after combustion and is superior in thermal stability and in producing and processing, is obtained. The gas generating composition for use in a pretensioner comprises a highly thermal stabilized binder, an oxidizing agent, and, if required, a chlorine-neutralizing agent as major ingredients. The gas generating composition can improve thermal stability of a gas generating agent, can reduce the amount of CO gases, is excellent in producing and processing, and can reduce chlorine-containing gases if required.