NOVELTY - A fuel cell system comprises hydrogen production apparatus (2) consists of modification part (21) which reform-reacts water vapor which is vaporized raw fuel and water to produce hydrogen-rich reformed gas and transformation part which reduces carbon monoxide left in the reformed gas by shift reaction, fuel cell stack which consumes reformed gas manufactured by hydrogen gas apparatus to produce electric power, and reformation water supply control unit (41) which controls amount of reformation water supply to carry out steam/carbon ratio (S/C).    USE - A fuel cell system (claimed).    ADVANTAGE - Reformed gas humidified by fuel cell stack is supplied by low-cost and simple control, without providing a separate humidifier, performance of fuel cell stack is fully improved, and durability is ensured.    DETAILED DESCRIPTION - A fuel cell system comprises hydrogen production apparatus (2) consists of modification part (21) which reform-reacts water vapor which is vaporized raw fuel and water to produce hydrogen-rich reformed gas and transformation part which reduces carbon monoxide left in the reformed gas by shift reaction, fuel cell stack which consumes reformed gas manufactured by hydrogen gas apparatus to produce electric power, and reformation water supply control unit (41) which controls amount of reformation water supply to carry out steam/carbon ratio (S/C), which is molar-flow-rate ratio of carbon in water vapor supplied to modification part and raw fuel and ranges from (F1x (T- Delta T)2+F2x (T- Delta T)+F3) to (F1x T2+F2x T+F3), where T is target operating-temperature ( degrees C) of fuel cell stack, Delta is allowable dew point fall temperature ( degrees C) of fuel cell stack, and F1-F3 is coefficient variably set from greater than or equal to 1 value among properties, reformation temperature in reforming reaction, or carbon monoxide concentration of transformation part.    DESCRIPTION OF DRAWING(S) - The drawing shows a block diagram of the control unit for the amount of water supply for reformation. (Drawing includes non-English language text).