NOVELTY - Supply of original fuel to a modifier (8) is restarted by opening a valve (10) when the temperature of the carbon monoxide removal portion (15) of the modifier is 50 degrees C or less. Then the pressure inside the modifier is maintained by closing he valve and stopping the supply of fuel to the heater (9) in the modifier simultaneously through another valve (12). The supply and stop of original fuel to modifier is repeated.    USE - For steam reforming of original fuel gas such as town gas for converting generated hydrogen rich gas into electric power.    ADVANTAGE - By supplying original fuel to modifier at the preset temperature of the combustion removal unit, generation of negative pressure inside the modifier is prevented and so deterioration of the catalyst is prevented. Since the fuel supply to the modifier is started and stopped repeatedly, the durability of the modifier and the fuel cell system is increased. Since the fuel supply to heater is stopped and the supply of process gas from the modifier to the fuel call is stopped simultaneously, the process gas is effectively burned by the combustor and the thermal efficiency of the fuel cell system is improved. The thermal storage tank also improves thermal efficiency.    DETAILED DESCRIPTION - The valve (12) supplies fuel to a heater, when the temperature of the modifier at the time of stoppage is at preset value. The heater heats the desulphurizing unit (2) which desulphurizes the sulfur content in the fuel for modification. A water vapor modifier (4) modifies the generated hydrogen rich gas using water vapor. A carbon monoxide (CO) transforming unit (6) decreases the carbon monoxide concentration in the hydrogen rich gas modified by the water vapor modifier. A removal device (7) decreases the carbon monoxide in the gas from the transforming unit. A valve (11) controls supply of water required for modification reaction. A combustor (17) stops supply of process gas to fuel cell (5) from the modifier, when the valve (12) is stopped. The thermal storage unit (19) which is hot water storage tank stores heat by heating the combustor through a heat exchanger (18). When the original fuel supply valve is stopped, a gas shut-off valve (14) provided at the exit of the modifier is also closed.    An INDEPENDENT CLAIM is also included for the stop system of fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the fuel cell system. (Drawing includes non-English language text).    Fuel cell system (1)    Desulphurizing unit (2)    Water vapor modifier (4)    Fuel cell (5)    CO transforming unit (6)    CO removing device (7)    Heater (9)    Original fuel supply valve (10)    Water supply valve (11)    Fuel supply valve (12)    Gas shut-off valves (13,14)    Carbon monoxide removal portion (15)    Temperature detector (16)    Combustor (17)    Heat exchanger (18)    Hot water storage tank (19)