PURPOSE: To provide a control method of reformer temperature capable of preventing the overheat of a reformer heat transfer part at the time of lowering the load of a fuel cell and circulating the whole quantity of CO2 gas from anode side to cathode side.

CONSTITUTION: This device is provided with a combustion air control valve 22, an air heater 24 by a combustion exhaust gas 8, a combusting air line 14 for heating and supplying an air 12 to the combustor of a reformer 10, and a temperature sensor 16 for detecting the temperature of the reformer heat transfer part. The temperature rise of the reformer heat transfer part is detected by the temperature sensor 16, or the combustion air control valve 22 is opened by a preceding signal based on load change command to excessively send the air in disregard of the flow rate at general time. Thus, the temperature of the combustion gas in the reformer 10 is lowered, and the overheat of the reformer heat transfer part is prevented.