NOVELTY - A carbon monoxide selective oxidation portion (36) which holds the heat exchanger, oxidizes the carbon monoxide in the hydrogen gas selectively to reduce carbon monoxide concentration in it. A heat exchanger (39) exchange the heat from the carbon monoxide content reduced hydrogen rich gas to hot water from a heat exchanger (32).    USE - For fuel cell apparatus (claimed) that produces EMF by electrochemical reaction involving hydrogen.    ADVANTAGE - The activity of reaction in carbon monoxide selective oxidation reaction is maintained sufficiently by cooling.    DETAILED DESCRIPTION - A reformation catalyst held in the reformer receives the hydrocarbon fuel such as natural gas or gasoline to be reformed to a hydrogen rich gas. The heat of the hydrogen rich gas ejected is exchanged with water in a heat exchanger held in the carbon monoxide selective oxidation unit. The carbon monoxide selective oxidation unit holds an oxidation catalyst to promote oxidation of carbon monoxide content in the hydrogen rich gas to reduce the carbon monoxide concentration in the gas. The carbon monoxide selective oxidation is cooled by hot water in the heat exchanger (39). The water is supplied in a gas-liquid two phase condition to the heat exchanger (39) by a water supply unit as an in-vitro water supply mode as the reformation reaction takes place at a higher temperature than the carbon monoxide selective oxidation reaction. A shift unit (34) is provided in the carbon monoxide selective oxidation unit to produce carbondioxide and hydrogen from the hydrogen rich gas. The reformation reaction occurs at more than 600degreesC, the shift reaction occurs in 200-400degreesC and the carbon monoxide selective oxidation reaction occurs in 100-200degreesC INDEPENDENT CLAIMS are also included for the following: (i) Hydrogen manufacturing method using the fuel reformer. (ii) Fuel cell apparatus equipped with fuel reformer for producing electromotive force (EMF) using electrochemical reaction involving hydrogen. (iii) Warming-up method of fuel reformer which involves using temperature of hydrogen rich gas for reheating.    DESCRIPTION OF DRAWING(S) - The figure shows an explanatory view representing the outline composition of the fuel cell apparatus. (Drawing includes non-English language text).    Heat exchangers (32,39)    Shift unit (34)    Carbon monoxide selective oxidation portion (36)