NOVELTY - A fuel processor comprises reformer (120), burner (130), carbon monoxide shifter (151), and medium path line (190). The reformer extracts hydrogen gas from a raw fuel by an extraction reaction with water. The burner heats up the reformer to a temperature for extracting the hydrogen gas. The carbon monoxide shifter removes carbon monoxide produced during the extraction reaction. The medium path line absorbs heat in a heat exchange medium. The medium path line passes the heat to the carbon monoxide shifter.    USE - A fuel processor for fuel cell system.    ADVANTAGE - The fuel processor reduces the time required to increase the temperature of carbon monoxide shifter to an operating temperature at early stage of start up since a rapid heating of carbon shifter is possible using the heat exchanger of steam. The fuel process has improved warming up structure.    DETAILED DESCRIPTION - A fuel processor comprises reformer (120), burner (130), carbon monoxide shifter (151), and medium path line (190). The reformer extracts hydrogen gas from a raw fuel by an extraction reaction with water. The burner heats up the reformer to greater than or equal to 100, preferably greater than or equal to 200degrees C for extracting the hydrogen gas. The carbon monoxide shifter removes carbon monoxide produced during the extraction reaction. The medium path line absorbs heat in water. The medium path line passes the heat to the carbon monoxide shifter. The medium path line is distinct from a fuel line for moving the hydrogen gas from the reformer to the carbon monoxide shifter. The fuel processor includes valves, and carbon monoxide remover. INDEPENDENT CLAIMS are included for:    (1) operating a fuel processor in which hydrogen gas is produced;    (2) generating hydrogen in a fuel processor; and    (3) fuel cell system.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of connection structure between reformer and carbon monoxide shifter of fuel processor.    Desulfurizer (110)    Reformer (120)    Burner (130)    Burner (150)    Carbon monoxide shifter (151)    Medium path line (190)