NOVELTY - Reformer comprises a reforming unit (220) for generating hydrogen and carbon monoxide by reacting hydrocarbon-based fuel with water, a water-gas shift (230) for primary removing carbon monoxide from the obtained hydrogen reacts and carbon dioxide, a first selective oxidation unit (240) for secondary removing carbon monoxide from carbon dioxide obtained by oxygen present in air supplied from the outside, and carbon monoxide present in the gases discharged from the water-gas shift, a waste heat recovering heat exchanger unit (250) provided with a waste heat recovering heat exchanger (251).    USE - The reformer is useful in fuel cell system (claimed).    ADVANTAGE - The reformer improves durability of fuel cell system, and is economical.    DETAILED DESCRIPTION - Reformer comprises a reforming unit (220) for generating hydrogen and carbon monoxide by reacting hydrocarbon-based fuel with water, a water-gas shift (230) for primary removing carbon monoxide from the obtained hydrogen reacts and carbon dioxide, a first selective oxidation unit (240) for secondary removing carbon monoxide from carbon dioxide obtained by oxygen present in air supplied from the outside, and carbon monoxide present in the gases discharged from the water-gas shift, a waste heat recovering heat exchanger unit (250) provided with a waste heat recovering heat exchanger (251) for recovering heat and the gases. The waste heat recovering heat exchanger is provided with a second selective oxidation unit for tertiary removing carbon monoxide form carbon dioxide which reacts with air supplied from the outside and carbon monoxide. The waste heat recovering heat exchanger and the second selective oxidation unit are located in single tubular body.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic representation of the reformer.    Reforming unit (220)    Water-gas shift (230)    First selective oxidation unit (240)    Waste heat recovering heat exchanger unit (250)    Waste heat recovering heat exchanger (251)