PURPOSE: To prevent overrise of the combustion temp. in a modifier by feeding back part of the anode exhaust gas on an anode exhaust gas circulating line when the load has sunk.

CONSTITUTION: The combustion temp. of a modifier 5 is measured by a thermometer 6. When the load sinks, a flow adjusting valve 30 an anode circulation exhaust gas feedback line 29 is opened upon a signal from a temp. adjuster 31 so that the combustion temp. of the modifier 5 does not exceed the set temp., and part of the anode exhaust gas on an anode exhaust gas circulation line 27 is fed back from the discharge side of an anode exhaust gas circulation blower 28 to between the incoming side of the blower 28 and an anode gas outgoing line 12. Thus circulation is made. Thereby overrise of the combustion temp. in the modifier 5 can be prevented without giving turbulence to a fuel cell concerned 1 and associate lines.