NOVELTY - The reformer useful in a fuel cell system, comprises a reforming reaction unit that converts a hydrogen-containing reforming fuel into a reforming gas primarily having hydrogen, a combustion reaction unit that heats the reforming reaction unit, and an oxygen supplying unit that supplies a gas to the combustion reaction unit. The oxygen supplying unit is a compressed gas container, a gas separation membrane or a pressure swing adsorption device. The oxygen supplying unit supplies a gas having an oxygen volume ratio of greater than 90% to the combustion reaction unit.    USE - The reformer is useful in a fuel system and for supplying the hydrogen to the electricity generating unit of a fuel cell system (claimed).    ADVANTAGE - The reformer ensures the reduction of time required to heat a reforming reacting unit to a catalyst temperature in an initial startup, generates hydrogen having a high purity more rapidly from the initial startup of a fuel cell, and highly enhances the actuation efficiency of the fuel cell.    DETAILED DESCRIPTION - The reformer useful in a fuel cell system, comprises a reforming reaction unit that converts a hydrogen-containing reforming fuel into a reforming gas primarily having hydrogen, a combustion reaction unit that heats the reforming reaction unit, and an oxygen supplying unit that supplies a gas to the combustion reaction unit. The oxygen supplying unit is a compressed gas container, a gas separation membrane or a pressure swing adsorption device. The oxygen supplying unit supplies a gas having an oxygen volume ratio of greater than 90% to the combustion reaction unit. A single fuel container provides the hydrogen-containing reforming fuel to the reforming reaction unit and the combustible fuel to the combustion reaction unit. The combustible fuel is identical in composition to the hydrogen-containing reforming fuel. The hydrogen-containing reforming fuel and the combustible fuel are butane and/or natural gas. The reforming reaction unit includes a catalyst having a predetermined catalytic temperature. The combustion reaction unit heats the reforming reaction unit to the catalytic temperature at start-up of the reformer and maintains the reforming reaction unit at or above the catalytic temperature during an operation of the reformer. A unit is arranged to remove the carbon monoxide contained in the reforming gas generated in the reforming reaction unit. The carbon monoxide removing unit includes a unit that performs a water gas shift reaction, and a unit that performs a preferential oxidation reaction. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows the fuel cell system using the reformer.    Reforming fuel container (110)    Combustible fuel container (111)    Reformer (130)    Electricity generating unit (140)    Air supplying unit. (150)