NOVELTY - A small cylindrical reformer includes a combustion unit, and a fuel-converting catalytic reaction unit that are structured as six cylindrical pipes (12, 16, 17, 18, 34, 35). Preferential oxidation reactor (43) is provided between first air fuel preheating passages (10) in the combustion part to decrease the concentration of carbon monoxide in the water gas shift reformate to a predetermined level or lower. The fuel-converting catalytic reaction unit has two reforming reaction parts, and a water gas shift reaction part. Each reforming reaction part has a reforming catalyst.    USE - Small cylindrical reformer for producing hydrogen from fossil fuels for a small fuel cell power plant.    ADVANTAGE - The small cylindrical reformer is structured to minimize heat loss and exhibit optimal heat exchange efficiency, has high heat efficiency despite having a compact size, is easily processed and manufactured, and simplifies a fluid stream flowing to minimize stagnation spaces, resulting in high efficiency, light weight, and ease of mass production.    DETAILED DESCRIPTION - A small cylindrical reformer comprises a combustion unit which includes a combustion part to burn air and fuel supplied from an air fuel inlet, a combustion gas distributor (15) to distribute the combustion gas burned in the combustion part, and a first combustion gas passage (20) and a second combustion gas (21) passage to guide the distributed combustion gas into an outlet to be discharged; and a fuel-converting catalytic reaction unit which includes two reforming reaction parts preparing a reforming reaction reformate, each having a reforming catalyst to reform feed and water supplied from a feed/water inlet, and a water gas shift reaction part preparing a water gas shift reformate, between the two reforming reaction parts to decrease a concentration of carbon monoxide in the reforming reacting reformate. The combustion reaction unit and the fuel-converting catalytic reaction unit are structured as six cylindrical pipes (12, 16, 17, 18, 34, 35) to realize optimal heat exchange efficiency. Preferential oxidation reactor (43) is provided between first air fuel preheating passages (10) in the combustion part to decrease the concentration of carbon monoxide in the water gas shift reformate to a predetermined level or lower.    DESCRIPTION OF DRAWING(S) - The drawing shows a longitudinal sectional view of the internal structure of a small cylindrical reformer.    Air fuel preheating passages (10)    Pipes (12, 16, 17, 18, 34, 35)    Combustion gas distributor (20, 21)    Superheater (33)    Preferential oxidation reactor (43)