To maintain the operating temperature of a fuel cell to the optimum temperature constantly, improve the plant efficiency of a system, and miniaturize and simplify the system, by installing a heating means, for increasing air temperature in a line, in the air supplying line from the air supply source of a fuel cell to a cathode electrode.

In air from a compressor 9, part of the air is branched to enter an auxiliary combustor 7, and the other most of the air enters an air supply line 18a. At that time, the temperature of the air rises by the working of a heating means 17. The air, passed through the line 18a, enters the cathode electrode 1b of a fuel cell 1, and the air, passed through the line 18a and an air supply line 18c, enters the combustion part 2b of a reformer 2. The air, supplied to the combustion part 2b, is burnt to be mixed with produced carbon dioxide, and passes through an air supply line 18d to enter the cathode electrode 1b. Moreover, the cathode electrode 1b discharges a cathode exhaust gas, and the whole quantity of the exhaust gas enters the auxiliary combustor 7 to drive an expansion turbine 8.