NOVELTY - The fuel processor comprises a reformer (110) that extracts hydrogen gas through a reaction between a hydrocarbon fuel source and water, a shift reactor (120) that transforms carbon monoxide (CO) included in a reformer gas discharged from the reformer to carbon dioxide by reacting CO with water, a combustion reactor (130) to contact an outer circumference of the shift reactor, a unit (140) for removing CO included in the reformer gas, a unit (150) that selectively supplies air to the shift reactor, the combustion reactor and the CO remover, and a first valve for controlling the oxygen supply.    USE - The fuel processor is useful for supplying hydrogen to an anode of a fuel cell system.    ADVANTAGE - The fuel processor ensures normal operation with reduced start up time by providing a shift reactor with an improved warming up structure to reduce an initial heating time of the shift reactor thus reducing the cost for re-starting.    DETAILED DESCRIPTION - The fuel processor comprises a reformer (110) that extracts hydrogen gas through a reaction between a hydrocarbon fuel source and water, a shift reactor (120) that transforms carbon monoxide (CO) included in a reformer gas discharged from the reformer to carbon dioxide by reacting CO with water, a combustion reactor (130) to contact an outer circumference of the shift reactor, a unit (140) for removing CO included in the reformer gas, a unit (150) that selectively supplies air to the shift reactor, the combustion reactor and the CO remover, and a first valve for controlling the supply of oxygen to the combustion reactor from the air supply unit, and a second valve for controlling the supply of the reformer gas discharged from the reformer to the shift reactor and the combustion reactor. The combustion reactor has a catalyst disposed along a gas flow channel formed in the combustion reactor. When the temperature of the shift reactor is above a normal operating temperature (200-300 degrees C), the second valve controls the reformer gas discharged from the reformer to be supplied to the shift reactor. Until the temperature of the shift reactor reaches the normal operating temperature, the first valve controls the supply of oxygen to the combustion reactor from the air supply unit to satisfy the equation, 0.1 less than volume of oxygen/(volume of hydrogen+volume of CO) less than 2. A third valve is present for controlling the supply of shift gas so that the shift gas discharged from the shift reactor is supplied to the CO remover and the combustion reactor. The supply of shift gas is controlled by the third valve when the temperature of the shift reactor is above an operable temperature (85-150 degrees C) and above or below the normal operating temperature. The combustion reactor has a cylindrical shape or a tube shape. An INDEPENDENT CLAIM is included for a method of operating a fuel processor.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of a configuration of a fuel processor.    Reformer (110)    Shift reactor (120)    Combustion reactor (130)    Carbon monoxide remover (140)    Air supply unit. (150)