NOVELTY - The reaction device (1) has an elliptical cross-sectioned honeycomb tube (4c) which holds a metal honeycomb catalyst used for selectively oxidizing combustible gas containing carbon monoxide, inside a heat exchanger. A rectangular reactor (4) of the device, is formed by alternately laminating some fins and the honeycomb tubes. Open ends of the tubes are connected to a separator (5d) in reactor.    USE - For selectively oxidizing carbon monoxide present in hydrogen-rich gas fed from reformer, to fuel cell system used as power source of electric vehicle, for removing carbon monoxide from hydrogen-rich gas.    ADVANTAGE - Increases the catalyst holding area per unit volume, by holding the catalyst in honeycomb tubes of elliptical cross-section. Since the catalyst is maintained at the temperature suitable for reaction to occur, eliminates the need to increase the number of reactors. Hence, reduces the size of the device. Enables perfect separation of two fluids as the open ends of the tube are connected to the separator. The thickness of the catalyst, enables homogenizing the temperature distribution in the catalyst. Hence, improves the efficiency of the reaction, and stabilizes the property of the catalyst. The rib enables uniformly conducting the heat produced by the reaction, and reinforcing the casing. Thermal deformation of casing is prevented and generation of temperature difference between the laminated tubes, is prevented, by provision of fins between adjacent tubes.    DETAILED DESCRIPTION - Thickness of the catalyst is 0.6-10 mm. The flow of fluid in the fin-side and honeycomb tube-side portions of the reactor, is countercurrent flow or parallel current flow. A rib is provided in the casing of the reaction device in which the heat exchanger removes the reaction heat produced by selective oxidation process. An air bleed hole of small diameter, is provided at the fin-side fluid inlet of the reactor.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of the heat-exchange type catalytic-reaction device. (Drawing includes non-English language text).    reaction device (1)    outlet (3)    reactor (4)    honeycomb tube (4c)    separator (5d)