NOVELTY - Reformer (30) comprises stacked reactors (31-35), each has respective channel.    USE - For use in fuel cell system (claimed) useful for producing electric power by generating electric energy via electrochemical reaction of oxygen and hydrogen.    ADVANTAGE - The inventive reformer is capable of improving the performance and efficiency of the entire fuel cell system with a simple structure.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) fuel cell system comprising a stack (10) for generating electrical energy y a reaction of hydrogen and oxygen, the reformer for generating hydrogen from fuel through a chemical catalytic reaction by thermal energy, a fuel supply assembly (50) for supplying fuel to the reformer, and an air supply assembly for supplying air to the reformer and the stack; and    (2) reforming method for the fuel cell structure comprising structuring a stack of reactors, supplying fuel and air to a first reactor and channeling the fuel and air through the first reactor to generate thermal energy, pre-heating the stack of reactors by the thermal energy, channeling a fuel and water mixture through the second reactor to be evaporated in the second reactor and transmitting a vaporized fuel and water mixture to the third reactor, channeling the vaporized fuel and water mixture through the third reactor, generating from the vaporized fuel and water mixture hydrogen gas-containing carbon monoxide supplying the hydrogen gas containing carbon monoxide to the fourth reactor, channeling the hydrogen gas containing carbon monoxide through the fourth reactor, performing a primary reduction of carbon monoxide concentration in the hydrogen gas, supplying hydrogen gas with reduced carbon monoxide concentration to the film reactor, injecting air into the fifth reactor, channeling the hydrogen gas with reduced carbon monoxide and the air through the fifth reactor, performing a secondary reduction of carbon monoxide concentration in the hydrogen gas, and exhausting hydrogen gas with a further reduced carbon monoxide concentration.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a fuel cell system.    Stack (10)    Reformer (30)    Reactors (31-35)    Fuel supply assembly (50)