NOVELTY - A fuel cell power generation system comprises a fuel cell being configured to feed a supply gas containing hydrogen gas to a fuel electrode, oxidize the supply gas, and discharge the residual gas as an exhaust gas. The supply gas comprises a mixed gas of a fuel gas containing hydrogen gas and all or part of the exhaust gas. The system further comprises a hydrogen gas separator having the function of separating hydrogen gas from the other gas. The system is so configured as to feed the supply gas to the fuel electrode via the hydrogen gas separator.    USE - Used as solid polymer electrolyte fuel cell, domestic cogeneration fuel cells, and vehicle-mounted and other movable fuel cell power generation systems.    ADVANTAGE - The system can always remove detrimental substances and maintain a high hydrogen concentration in response to change in fuel gas composition and to contamination of impurities upstream of the fuel cell, can simultaneously recover unreacted hydrogen in the fuel cell outlet and recirculate the recovered hydrogen to the cell inlet, has a simple configuration, and can be easily operated. Thus, substantially 100% of the fuel (hydrogen gas) can be utilized, and the generation efficiency is improved. The system also eliminates the need of carbon monoxide-shift converter necessary in conventional equivalents and eliminates the need of operation control for elevating and maintaining the temperature of the shift converter catalyst. This results in shorter time for system startup and reduced energy consumption upon startup. Thus, the generation efficiency and the reliability of the system in operation control are improved.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is also included for operating a fuel cell, comprising:    (A) mixing a fuel gas conveyed from a fuel feed unit with an exhaust gas discharged from a fuel electrode of a fuel cell;    (B) pressurizing the resulting mixed gas and feeding the pressurized mixed gas to a unit for separating hydrogen gas; and    (C) feeding the separated hydrogen gas to the fuel electrode of the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure is a schematic diagram showing the configuration of a fuel cell power generation system.    Mixer (3)    Circulation blower (5)    Bypass piping (17)    Pressure regulator (20)    Cooler (27)    Raw fuel feed unit (28)