To improve the reliability and efficiency of a fuel gas generating system.

Fuel gas is generated by isolating only hydrogen using a hydrogen-isolating membrane from a hydrogen-containing mixed gas which is generated by reforming reaction of a raw material such as alcohols. Hydrogen permeability is improved by supplying purge gas to the hydrogen isolating part to decrease the hydrogen partial pressure. Then, the total pressure of materials, purge gas, and fuel gas is controlled in response to the required power of the fuel battery. When the required power is low, the total pressure is lowered to save the energy consumption. When the required power is high, improvement of hydrogen permeability is attempted by increasing the total pressure so that the hydrogen partial pressure difference on the hydrogen isolating membrane is enlarged. When carbon monoxide is leaked to the hydrogen isolating part, the total pressure of the isolating pat is increased to control the leakage.