To provide a method for improving the efficiency and for reducing an exhaust gas in a fuel cell system.

In the fuel system wherein air is fed to the negative electrode side and hydrocarbon is fed to the positive electrode side in operation, the oxygen content of the fed air and the hydrogen content of a combustible gas are increased by condensation while individually using gas condensation methods; reforming of hydrocarbon is executed by the condensation method on the positive electrode side, and its reaction gas is separated to be successively used differently as follows: hydrogen is fed to one or a plurality of low-temperature fuel cells; water and hydrocarbon are condensed and led back to a reformer process; and carbon dioxide and carbon monoxide are fed to exhaust gas catalyst; and the gases are changed into carbon dioxide and molecule nitrogen and emitted to the atmosphere/air.