NOVELTY - A fuel is burned by combustion unit (27) of fuel cell system having modifier (20), fuel supply unit (22), water supply unit (24), carbon monoxide reduction unit (34), fuel cell and heating unit. Heating unit is operated during control of modifier to preset temperature (T1), at which modification is carried out. The unit (34) is heated to temperature (T2), at which reduction is carried out. Water is supplied to modifier, when reduction unit reaches temperature (T2). Fuel is supplied to modifier, and reformed gas is generated. Thus, the fuel cell system is started.    USE - Starting of fuel cell system used as electric power supply.    ADVANTAGE - The fuel cell system is safely, reliably and efficiently started, and is compact and economical.    DETAILED DESCRIPTION - A fuel of predetermined amount is burned by combustion unit (27) of fuel cell system. The fuel cell system has modifier (20) for generating reformed gas by modifying fuel, unit (22) for supplying fuel to modifier, unit (24) for supplying water to modifier, unit (34) for reducing concentration of carbon monoxide contained in reformed gas, fuel cell generated using reformed gas and heating unit for heating carbon monoxide reduction unit. The heating unit is operated during control of modifier to preset temperature (T1), at which modification is carried out. The carbon monoxide reduction unit is heated to temperature (T2), at which reduction is carried out. Water is supplied to modifier, when the reduction unit reaches the temperature (T2). Fuel is supplied to modifier, and reformed gas is generated after predetermined time. Carbon monoxide contained in reformed gas is reduced, and gas is supplied to fuel cell. The unconverted gas discharged from fuel cell is burned by combustion unit. Thus, the fuel cell system is started. An INDEPENDENT CLAIM is included for fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows the systematic diagram of fuel cell system. (Drawing includes non-English language text)    Modifier (20)    Fuel supply unit (22)    Water supply unit (24)    Combustion apparatus (27)    Carbon monoxide reduction unit (34)