NOVELTY - The system comprises an internal tank (1) for storing the liquid fuel and a pump (4) connected to the tank for pressurizing the fuel. A preheater (5) is connected to pump, to pre-heat the fuel with combustion gas. An internal tank, pump, preheater, desulfurizer (6), a pressure control valve (7), and a fuel supply valve (8) are connected in series. A pressure relief valve (9) and a safety valve (10) are connected in parallel to pressure control valve. A bypass valve (11) is connected in parallel to internal tank and downstream of pressure relief valve and safety valve.    USE - Liquid-fuel desulfurization system of polymer electrolyte fuel cell system using liquid fuel e.g. kerosene, light oil, naphtha, gasoline etc.    ADVANTAGE - The decline in the electric power generation efficiency can be suppressed, without reducing the supply of power of fuel cell system directly and the temperature of desulfurizer can be maintained at high temperature.    DETAILED DESCRIPTION - A heat exchanger is provided to heat exchange the air for combustion between the desulfurizer and pressure control valve. A cylindrical container is filled with the desulfurization catalyst and a pressure resistant cap is provided to close the upper and lower sides of the container. A metal-mesh is fixed with the desulfurization catalyst from the upper and lower sides of the container. The combustion flow path covers the outer periphery of the side of the cylindrical container. An INDEPENDENT CLAIM is included for operating method of liquid fuel desulfurization system which involves driving the liquid fuel desulfurization system after pressurizing the desulfurizer and stopping the pump operation. At the time of stop, the fuel supply valve is closed and the desulfurizer is pressure-reduced. Then low sulfur liquid fuel is stopped in this state and the pump operation is started again.    DESCRIPTION OF DRAWING(S) - The drawing shows an explanatory view of the liquid fuel desulfurization system of polymer electrolyte fuel cell system. (Drawing includes non-English language text)    Internal tank (1)    Pump (4)    Preheater (5)    Desulfurizer (6)    Pressure control valve (7)    Fuel supply valve (8)    Pressure relief valve (9)    Safety valve (10)    Bypass valve (11)