A fuel cell system including a fuel cell module and a control device; the fuel cell module comprising: a fuel cell stack formed by stacking a plurality of fuel cells for generating electricity by electrochemical reactions of a fuel gas and an oxygen-containing gas; a partial oxidation reformer for reforming a mixed gas of the raw fuel chiefly containing hydrocarbon and an oxygen-containing gas; a steam reformer for reforming a mixed gas of the raw fuel and water vapor; an evaporator for evaporating water and supplying the water vapor to the steam reformer; and a heat exchanger for raising the temperature of the oxygen-containing gas by heat exchange with a combustion gas and supplying the oxygen-containing gas to the fuel cell stack; the control device comprising: a carbon amount determination unit for determining an amount of carbon in the fuel gas supplied to the fuel cell stack depending on a required output of the fuel cell stack; a temperature detector unit for detecting a temperature of the steam reformer and a temperature of the evaporator; an S/C determination unit for determining a range of steam/carbon ration based on the temperature of the steam reformer; a water supply amount determination unit for determining a range of the water supply amount to the evaporator based on the amount of carbon and the steam/carbon ratio; an evaporator operating state determination unit for determining whether or not the temperature of the evaporator is a temperature that corresponds to an amount of water that is within the range determined, by the water supply amount determining unit; and a reformer control unit for controlling the steam reformer and the partial oxidation reformer based on the determination result of the evaporator operating state determination unit, characterized in that the steam reformer is directly attached to one end of the heat exchanger. a fuel cell stack formed by stacking a plurality of fuel cells for generating electricity by electrochemical reactions of a fuel gas and an oxygen-containing gas; a partial oxidation reformer for reforming a mixed gas of the raw fuel chiefly containing hydrocarbon and an oxygen-containing gas; a steam reformer for reforming a mixed gas of the raw fuel and water vapor; an evaporator for evaporating water and supplying the water vapor to the steam reformer; and a heat exchanger for raising the temperature of the oxygen-containing gas by heat exchange with a combustion gas and supplying the oxygen-containing gas to the fuel cell stack; a carbon amount determination unit for determining an amount of carbon in the fuel gas supplied to the fuel cell stack depending on a required output of the fuel cell stack; a temperature detector unit for detecting a temperature of the steam reformer and a temperature of the evaporator; an S/C determination unit for determining a range of steam/carbon ration based on the temperature of the steam reformer; a water supply amount determination unit for determining a range of the water supply amount to the evaporator based on the amount of carbon and the steam/carbon ratio; an evaporator operating state determination unit for determining whether or not the temperature of the evaporator is a temperature that corresponds to an amount of water that is within the range determined, by the water supply amount determining unit; and a reformer control unit for controlling the steam reformer and the partial oxidation reformer based on the determination result of the evaporator operating state determination unit, characterized in that the steam reformer is directly attached to one end of the heat exchanger.