NOVELTY - Reformer (100) comprises a reforming reactor having a catalyst capable of performing an auto thermal reforming (ATR) catalyst reaction and a steam reforming (SR) catalyst reaction. A heat source contacts one side of the reforming reactor and provides the reforming reactor with heat. An air feeder feeds the reforming reactor with air by an air flow control unit, which has valve before the reactor. It feeds the heat source with air and the heat source generates heat by burning combustion fuel.    USE - For use in fuel cell system useful for generating power.    ADVANTAGE - The reformer has a catalyst capable of performing an auto-thermal catalyst reaction and a steam reforming catalyst reaction according to the type of reaction material introduced into the reforming reactor. The ATR catalyst reaction featuring in a short preheating time is performed while the reformer is initially driven, so that hydrogen can be produced when the reformer is initially operated, thus efficiently operating the fuel cell. The process of reducing carbon monoxide is performed individually in the ATR catalyst reaction and the SR catalyst reaction, thus efficiently producing hydrogen with high purity.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) fuel cell system comprising a fuel feeder to store and supply raw fuel, the reformer to generate hydrogen from the raw fuel, a blower to supply air, an electric generator to generate electricity by an electrochemical reaction between hydrogen and oxygen in air, and a water storage container to supply water to the reformer; and    (2) method for operating a reformer comprising producing hydrogen by an auto-thermal reforming catalyst reaction while reaction material including fuel, water and air containing oxygen is supplied to a reforming reactor, and stopping the supply of air containing oxygen thus starting a steam reforming catalyst reaction.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view of a proton exchange membrane fuel cell (PEMFC) system utilizing the reformer.    Reformer (100)    Fuel feeder (110)    Fuel mixer (120)    Electric generator (140)    Blower (150)