To enhance plant efficiency, by providing a device for reforming fuel gas into hydrogen-contained anode gas in place of a reformer.

This device is equipped with a fuel cell 20 composed of a cathode and anode for generating power from oxygen-contained cathode gas and hydrogen-contained anode gas, a fuel preheater 36 for burning part of cathode exhaust gas exhausted from the cathode and anode exhaust gas exhausted from the anode and heating steam-contained fuel gas by heat thereby generated, a mixer 38 for partially oxidizing the heated fuel gas from the fuel preheater 36 by adding oxygen to it and supplying the hydrogen-contained anode gas to the anode, and a circulation line 3 for mixing combustion exhaust gas of the fuel preheater 36 and air into part of the cathode exhaust gas and circulating it through the cathode.