PURPOSE: To provide internal modification performed on a fuel electrode simultaneously as fuel modification and power generation reaction by providing functions of modifying hydrocarbon of C2 or more in city gas, and controlling a methane reaction ratio easily.

CONSTITUTION: Modifying gas and air flowing from a tube 15 through an air blower 8 and a heating air preheater 9 are supplied from a tube 23 to a solid electrolyte fuel cell 10 to generate power at 1000°C, and temperature of fuel discharge gas getting into a combustor 11 also becomes about 1000°C. Next, recycle gas of 1000°C inside a tube 17 supplies heat to a reaction tube part 7b of a low temperature for heat absorption reaction to lower the temperature of the recycle gas itself to be low-temperature recycle gas of 500-800°C, which is then supplied from a tube 22 through a recycle gas flow regulation valve 6 to an ejector 5. Hydrocarbon of C2 or more in city gas is thus modified in the reformer 7, thereby methane reformation reaction can be controlled.