NOVELTY - Small cylindrical reformer comprises: combustion unit; and fuel-converting catalytic reaction unit with two parts (37, 38) to form reforming reaction reformate, and part (39) to form water gas shift reformate, between two reforming parts to decrease carbon monoxide in reformate; where units form six pipes; routes for water gas shift reformate and oxidation reactor (43), and feed overlap to increase heat exchange; and oxidation reactor is at lower portion to decrease carbon monoxide in water gas shift reformate, and increase heat transfer of fuel preheating passage (10) and fuel inlet.    USE - As small cylindrical reformer (claimed), for production of hydrogen from fossil fuels for small fuel cell power plant, home or power plants, power supply sources for fuel cell automobiles, ammonia synthesis, methanol synthesis, petroleum refining (e.g. dehydrogenation, hydrogenation), general and fine chemical industries, electronic and semiconductor industries, and food and metal processing industries.    ADVANTAGE - The cylindrical reformer provides small cylindrical reformer, in which reforming reactor, water gas shift reactor (high temperature or low temperature), combustor, heat exchanger, and steam generator; are integrated in single container to form optimal heat exchanging network able to exhibit maximal performance of each constituent, minimize heat loss, and optimize heat exchange efficiency, so that the reformer can be easily processed and manufactured to have compact size, while resulting in high efficiency, light weight and ease of mass production. The reformer thus has high heat efficiency despite having compact size, and further simplifies fluid stream flowing in it to minimize stagnation spaces. In the reformer, in heat transfer efficiency, optimal heat efficiency can be obtained via heat exchange between greater than or equal to 2 fluids, while conducting combustion reactions in presence of catalyst suitable for each reaction. These constituents may be integrated in single reactor so that resultant apparatus is structured to have a compact size while minimizing heat loss. In addition, six pipes constituting apparatus are designed to be integrated, and thus can be easily processed and manufactured even while having high heat efficiency. Hence, when reformer is applied to fuel cell power system, total power efficiency can be increased. Moreover, since reformer is lightweight, it can be promptly operated according to initial start-up time or operation capacity change. Also, reformer can be favorably commercialized through designs able to easily realize mass production and durability. When such a reformer is operated, methane conversion rate is greater than or equal to 94%, CO content after water gas shift reaction is less than or equal to 0.7%, CO content after preferential CO oxidation in hydrogen (PROX) is less than or equal to 10 ppm, and heat transfer efficiency is high.    DETAILED DESCRIPTION - A small cylindrical reformer comprises: combustion reaction unit, which includes combustion part burning air and fuel supplied from air/fuel inlet (1), combustion gas distributor (15) distributing combustion gas generated in combustion part, and first combustion gas passage (20) and second combustion gas passage (21) to guide distributed combustion gas into outlet (2) to be discharged; and fuel-converting catalytic reaction unit including two reforming reaction parts (37, 38) preparing reforming reaction reformate, each having reforming catalyst to reform feed and water supplied from feed/water inlet (3); and water gas shift reaction part (39) preparing water gas shift reformate, between two reforming reaction parts to decrease concentration of carbon monoxide in reforming reaction reformate; where combustion reaction unit and fuel-converting catalytic reaction unit are structured as six cylindrical pipes (12, 16, 17, 18, 34, 35) to realize optimal heat exchange efficiently; route supplying water gas shift reformate and preferential oxidation reactor (43), and route supplying feed and water into reforming reaction parts from feed/water inlet overlap each other at lower portion of reformer so as to increase heat transfer efficiency; and preferential oxidation reactor is provided at lower portion of reformer to decrease concentration of carbon monoxide in water gas shift reformate to predetermined level or lower, and increase heat transfer efficiency with air/fuel preheating passage (10) communicating with air/fuel inlet. The preferential oxidation reactor includes sheet-shaped air/fuel distributor for heat exchange with air and fuel supplied from air/fuel inlet. The preferential oxidation reactor and air/fuel preheating passage provided at lower portion of reformer are removably attached to small reformer.    DESCRIPTION OF DRAWING(S) - The figure shows longitudinal sectional view of internal structure of the small cylindrical reformer.    Air/fuel inlet (1)    Outlet for distributed combustion gas (2)    Feed/water inlet (3)    Air (4)    Outlet for water gas shift reformate (5)    First air/fuel preheating passage (10)    Burner (11)    First pipe/combustion chamber (12)    Combustion gas distributor (15)    Second pipe (16)    Third pipe radially positioned between first pipe and fourth pipe (17)    Fourth pipe surrounding reactor (18)    First and second combustion gas passages (20, 21)    Exhaust gas passage (23)    Fifth pipe (34)    Sixth pipe (35)    Reforming reactor (36)    First and second reforming reaction reformate passage (37, 38)    Water gas shift reactor (39)    First and second water gas shift reformate passages (40, 41)    Water gas shift reformate/air passage (42)    Oxidation reactor (43)