NOVELTY - A reformer (20) comprises a reforming section (21); pumps (45, 42) for supplying combustible gas to a combustion section (25); a combustion air pump (65) for supplying air to combustion section; temperature sensor (21c) for detecting combustion state of combustion section; valves (46, 66) for changing the quantity of combustible gas and/or air supplied to combustion section; a combustion state change detector responsive to a detection result from temperature sensor for detecting a change caused by change of the supply quantity through valve; and a combustion state checking unit.    USE - As a reformer for a fuel cell system (claimed) useful in electric power generation.    ADVANTAGE - With the construction of the reformer, it is possible to change the supply quantity of combustible gas and/or combustion oxidizing gas supplied to the combustion section. Since the change in the combustion state of the combustion section resulting from the change in the supply quantity can be detected based on the detection result from the temperature sensor, it is possible to check the combustion state of the combustion section which arose before the change by the valves, based on the detection result. Accordingly, without using expensive sensors, it is possible to detect the state of the combustion section being likely to go out, reliably at a low cost. Since the fuel cell system is provided with the reformer which is capable of reliably detecting at a low cost the state of the combustion section being likely to go out, it is possible to make the entire system low in cost and, to reliably detect the state of the combustion section being likely to go out. As a consequence, the efficiency of the fuel cell system in electric power generation can be enhanced with the reformer operating in an optimum combustion state.    DETAILED DESCRIPTION - A reformer (20) comprises a reforming section (21) for generating hydrogen-containing fuel gas by reforming reformed fuel; a combustion section (25) for burning combustible gas with combustion oxidizing gas (air) to heat the reforming section with resultant combustion gas; pumps (45, 42) for supplying the combustible gas to the combustion section; a combustion air pump (65) for supplying the combustion oxidizing gas to the combustion section; temperature sensor (21c) for detecting the combustion state of the combustion section; valves (46, 66) for changing the supply quantity of at least either one of the combustible gas and the combustion oxidizing gas supplied to the combustion section; a combustion state change detector responsive to a detection result from the temperature sensor for detecting a change which is caused in the combustion state of the combustion section as a result that the change of the supply quantity is brought by the valve; and combustion state checking unit responsive to a detection result from the combustion state change detector for checking the combustion state which arose at the combustion section before the change made by the valve. An INDEPENDENT CLAIM is included for a fuel cell system comprising the reformer; and a fuel cell (10) having a fuel pole (11) to which the reformed gas generated in the reformer is supplied as fuel gas; where at least either one of combustion fuel supplied from the outside of the system, and the reformed gas led from the reformer and fuel gas offgas led from the fuel pole of the fuel cell is supplied as combustible gas to the combustion section of the reformer.    DESCRIPTION OF DRAWING(S) - The figure shows schematic view of a fuel cell system incorporating a reformer.    Reformer (20)    Reforming section (21)    Temperature sensor (21c)    Combustion section (25)    Reformed fuel pump (42)    Combustion fuel pump (45)    Combustion fuel valve (46)    Combustion air pump (65)    Combustion air valve. (66)