NOVELTY - The fuel cell comprises a reforming apparatus (20) to perform reformation reactions to produce a fuel gas and a heat by-product, a stack (40) to receive the fuel gas from the reforming apparatus and to generate energy, a preheating unit to heat the stack, an operation control unit to supply the fuel gas or heat by-product to the preheating unit during an initial operation of the reforming apparatus and to supply the fuel gas to the stack upon the completion of the initial operation, and a heat exchanger. The reforming apparatus includes a heating burner (22), and a reformer (23).    USE - The fuel cell is useful for generating electricity.    ADVANTAGE - The fuel cell rapidly elevates an internal temperature in an economical manner until the fuel cell initially operates and then enables normal operation, and can be simply and economically manufactured with high efficiency.    DETAILED DESCRIPTION - The fuel cell comprises a reforming apparatus (20) to perform reformation reactions to produce a fuel gas and a heat by-product, a stack (40) to receive the fuel gas from the reforming apparatus and to generate energy, a preheating unit to heat the stack, an operation control unit to supply the fuel gas or heat by-product to the preheating unit during an initial operation of the reforming apparatus and to supply the fuel gas to the stack upon the completion of the initial operation, and a heat exchanger. The reforming apparatus includes a heating burner (22), a reformer (23), which is heated by the heating burner, a shift reactor (26) to receive gas discharged by the reformer, a unit (30) to mix the gas received from the shift reactor with external air, and a unit (28) for removing carbon monoxide from mixture of gas and external air to provide the fuel gas. The stack includes: cells having a fuel electrode to receive the fuel gas through a first passage extended from the carbon monoxide remover and an air electrode, where the fuel electrode and air electrode are positioned at opposite sides of an electrolyte film; cooling plates to receive circulation water from a circulation water reservoir, where each cooling plate is interposed between adjacent cells; and end plates provided at an outermost edge of the stack, where the stack generates electricity energy. The operation control unit includes a controller to control the mixer and to determine a supply passage of the fuel gas. The preheating unit is a circulation water reservoir or a combustion reactor used to heat the stack during the initial operation. The combustion reactor includes a catalyst acceptor containing a catalyst to react with the fuel gas to generate heat. The combustion reactor communicates with a second passage extended from the mixer, takes the shape of a pocket encompassing the outside of the circulation water reservoir or an end plate, takes the shape of a tube wound along the external surface of the circulation water reservoir or the end plate, and further comprises a third passage, where the fuel gas passes through the fuel electrode or the air electrode. The heat by-product is warm water passed through the heat exchanger. The water is warmed by a gas discharged from a heating burner or a gas discharged from the reformer. The circulation water reservoir is provided with a fourth passage, where the warm water is supplied through the fourth passage. The second passage includes a fifth passage branched toward the combustion reactor from a control valve arranged on the second passage, and a sixth passage branched toward the heating burner from the control valve. The initial operation includes a first operation in which the temperature of the shift reactor is lower than 100 degrees C, and a second operation in which the temperature of the stack is lower than 120 degrees C. The warm water as the thermal by-product is supplied to the preheating unit during the first operation when the temperature of the reformer is higher than 500 degrees C. The gas generated at the reforming apparatus is supplied to the preheating unit during the second operation. The external air has a composition in which a ratio (v/v) of oxygen to the sum of hydrogen and carbon monoxide is 0.1-2. An INDEPENDENT CLAIM is included for a method for controlling a fuel cell including a reforming apparatus and a stack.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an operation mechanism of a fuel cell.    Reforming apparatus (20)    Heating burner (22)    Reformer (23)    Shift reactor (26)    Carbon monoxide remover (28)    Mixer (30)    Stack. (40)