NOVELTY - The selective partial oxidation reactor (8) has heat exchanger (37) to exchange heat between the gas introduced through the gas passageway (34) and water in the heat carrier route (35). A catalyst is provided in the heat exchanger.    USE - For fuel cell, hydrogen engine.    ADVANTAGE - The selective partial oxidation reactor is thermally stabilized in a short period by maintaining the constant temperature inside the reactor. Starting period of fuel cell system is shortened and starting property is improved. Mixture of gas in the passageway is accelerated with the heat transfer fins. Durability of the catalyst is improved by providing coating film of catalyst on the alumina. Structure of fuel cell system is simplified by having water as heat carrier and heat exchanging efficiency is increased. Selectivity of reaction is improved in the low carbon monoxide concentration area. Leakage of water from heat carrier route is detected easily and the reliability of fuel cell system is improved. Stopping of combustion reaction of carbon monoxide inside the reactor is prevented by maintaining the temperature of water in the heat carrier route at at least 60 degrees C.    DETAILED DESCRIPTION - Aluminum made heat transfer fins (38) are provided in the heat exchanger. Catalyst coating film (39) is provided on the alumina which is formed by the oxidation of the heat exchanger surface. The catalysts are selected from any one of the Ru group, Pt group, Rh group, Au group and Co group. The gas and the water in the gas passageway and heat carrier route flow in reverse direction mutually. Water leak detectors are provided between the gas passageway and the heat carrier route to detect heater leak from the heat carrier route. Warm water of at least 60 degrees C flows in the heat carrier route.    DESCRIPTION OF DRAWING(S) - The figure shows the sectional view of selective partial oxidation reactor.    Oxidation reactor (8)    Gas passageway (34)    Heat carrier route (35)    Heat exchanger (37)    Heat transfer fins (38)    Coating film (39)