NOVELTY - A fuel cell system has modifier (3) which modifies raw fuel and produces fuel gas, fuel cell (21) which has an anode (24) where produced fuel gas is supplied and a cathode (25) where oxygen gas is supplied, combustion part (22) where the fuel component in the discharged fuel gas and the discharged oxygen gas after a power generation reaction are burned, output adjustment part (L) which adjusts the output current of the fuel cell, fuel gas supply amount adjustment part (F), and operation control part (C).    USE - A fuel cell system.    ADVANTAGE - Change of output current is increased while stably maintaining the combustion.    DETAILED DESCRIPTION - A fuel cell system has modifier (3) which modifies raw fuel and produces fuel gas, fuel cell (21) which has an anode (24) where produced fuel gas is supplied and a cathode (25) where oxygen gas is supplied, combustion part (22) where the fuel component in the discharged fuel gas from the anode and the discharged oxygen gas from the cathode after a power generation reaction are burned, and which heats the modifier using the produced combustion heat, output adjustment part (L) which adjusts the output current of the fuel cell, fuel gas supply amount adjustment part (F) which adjusts the supply amount of the fuel gas from the modifier to the anode, and operation control part (C) which controls the operation of the output adjustment part and fuel gas supply amount adjustment part. Operation of the output adjustment unit is controlled and the output current is adjusted according to the characteristic curve which has defined the target value of the fuel utilization which is the ratio of the quantity of the fuel gas used for power generation reaction with the anode to the quantity of the fuel gas supplied to the anode as a function of the output current. The operation of the fuel gas supply amount adjustment part is controlled, and supply amount of the fuel gas from the modifier to the anode is adjusted. When carrying out the increase change of the output current toward a predetermined target electric current, operation of the fuel gas supply amount adjustment part is controlled, and supply amount of the fuel gas of the modifier to the anode is increased. The fuel utilization was reduced rather than the target value of the fuel utilization during the increase start defined by the characteristic curve. After that, operation of the output adjustment part and the fuel gas supply amount adjustment part is controlled. Transient time driving for changing toward the target value of the fuel utilization during completion of increase in which the output current and the fuel utilization are provided with the function of the target electric current according to the target electric current and characteristic curve is performed.    DESCRIPTION OF DRAWING(S) - The drawing shows a cross-sectional view of the fuel cell system. (Drawing includes non-English language text).    Modifier (3)    Fuel cell (21)    Combustion part (22)    Anode (24)    Cathode (25)    Operation control part (C)    Fuel gas supply amount adjustment part (F)    Output adjustment part (L)