NOVELTY - A fuel cell system comprises a first heating mechanism (77a) for supplying some of an exhaust gas discharged from a fuel cell stack (12) to the reformer as a heat source for directly heating the reformer; a second heating mechanism (77b) for supplying the remaining exhaust gas to the heat exchanger as a heat source for indirectly heating the reformer; and a valve control mechanism for adjusting valve mechanism based on the temperature of the reformer to maintain the state of the reformer to have a predetermined reforming condition value.    USE - For e.g. a vehicle.    ADVANTAGE - The reformer is directly heated by some of the exhaust gas and indirectly heated by the heat exchanger to which the remaining exhaust gas is supplied. By the direct heating and/or the indirect heating using the exhaust gas, the state of the reformer is maintained to have the predetermined reforming condition values reliably. The steam/carbon (S/C) ratio may be lowered down to the coking limit. The amount of supplied water is reduced and the load of the water supply power source, e.g. the water pump, is reduced efficiently. By simply operating the single valve mechanism, the state of the reformer is maintained to have predetermined reforming condition values. With the simple structure and process, the reforming reaction is carried out to a greater extent. By utilizing the exhaust gas as the heat source for directly and/or indirectly heating the reformer, significant improvement in the recovery rate of the waste heat is achieved economically.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell stack formed by stacking fuel cells each including an electrolyte electrode assembly and a separator stacked together, where the electrolyte electrode assembly includes an anode, a cathode and an electrolyte interposed between the anode and the cathode; a heat exchanger for heating an oxygen-containing gas to be supplied to the fuel cell stack; a reformer for reforming a mixed fuel of a raw fuel containing hydrocarbon and water vapor to produce a reformed gas; a first heating mechanism for supplying some of an exhaust gas discharged from the fuel cell stack after consumption in power generation reaction to the reformer as a heat source for directly heating the reformer; a second heating mechanism for supplying the remaining exhaust gas to the heat exchanger as a heat source for heating the oxygen-containing gas and supplying the heat generated in the heat exchanger to the reformer has a heat source for indirectly heating the reformer; a temperature detection unit (130a) for detecting the temperature of the reformer; a valve mechanism (132) for adjusting the flow rate of the exhaust gas supplied to the first heating mechanism; and a valve control mechanism for adjusting the valve mechanism based on the temperature of the reformer detected by the temperature detection unit to control the ratio between the amount of heat supplied from the first heating mechanism to the reformer and the amount of heat supplied from the second heating mechanism to the reformer in order to maintain the state of the reformer to have a predetermined reforming condition value.    An INDEPENDENT CLAIM is also included for a method of operating a fuel cell system comprising    (A) supplying some of an exhaust gas discharged from the fuel cell stack after consumption in power generation reaction to the reformer as a heat source for directly heating the reformer;    (B) supplying the remaining exhaust gas to the heat exchanger as a heat source for heating the oxygen-containing gas and supplying the heat generated in the heat exchanger to the reformer as a heat source for indirectly heating the reformer;    (C) detecting the temperature of the reformer; and    (D) adjusting the flow rate of the exhaust gas to be supplied to the reformer based on the detected temperature of the reformer to maintain the state of the reformer to have a predetermined reforming condition value.    DESCRIPTION OF DRAWING(S) - The figure shows a diagram of a fuel cell system.    Fuel cell stack (12)    Fluid unit (14)    Casing (16)    First heating mechanism (77a)    Second heating mechanism (77b)    Inlet (102)    Outlet (122)    Temperature detection unit (130a)    Valve mechanism (132)