TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Apparatus: The cooling means is a water cooling means and the heating means utilizes reaction heat generated by the reaction of hydrogen and CO with the oxidant gas. The upstream side of the catalyst layer is formed from a different material to the downstream side and the downstream catalyst is of lower activity than the upstream catalyst. The catalyst is supported by a metallic material. The amount of oxidant gas is changed in correspondence with the temperature of the catalyst layer. A flow pathway of the reformed gas is formed at or close to the catalyst layer via a partition, so as to heat the downstream side of the catalyst layer by contact with the gas before it passes through the cooling means, and contact is preferably in an opposing direction to stream passing through cooling means. A reaction segment, preferably tube shaped, is placed on the periphery of the flow pathway or in the flow pathway such that flowpath is formed around the segment. Two or more reaction segments may be connected in parallel. A branched pathway is formed which bifurcates downstream from a connection between the reformed gas supply pathway and the oxidant gas supply pathway and is connected to the reaction segment at the middle point of the catalyst layer, the branched pathway acting as the cooling or heating means. The apparatus further comprises means for changing the cross-sectional area of the reformed gas and branched pathways at their connection to control the amount of reformed gas supplied to the reaction segment. The reaction segment comprises at least a two segmented catalyst layer and at least one uppermost catalyst layer has a a part with no catalytic function or a part with low reactivity to CO. The uppermost catalyst layer is comprised of a catalyst pellet and the lowermost layer has the shape of a honeycomb. Alternatively the reaction segment comprises a two segmented honeycomb shaped catalyst in which the uppermost layer is larger than the lowermost layer with respect to open area. The uppermost catalyst layer comprises platinum group metal supported by alumina group metal and the lowermost catalyst layer comprises a platinum group metal supported on a zeolite material. The uppermost catalyst layer comprises a catalyst which has been treated at a higher temperature than the lower layer. The catalyst layer comprises a temperature measurement apparatus segment and a means for controlling the supply of oxidant gas according to the temperature. EXAMPLE - An alumina carrying platinum was coated on a cordierite honeycomb of 50 mm diameter and 100 mm length to form the catalyst layer 1. The catalyst layer 1 thus formed was placed in the reaction chamber 9 of a hydrogen purifying apparatus. A reformed gas containing 1 % CO, 15% carbon dioxide, 15% water vapor and remainder hydrogen was introduced into the apparatus through a reformed gas inlet 2 at a rate of 10 I/min. Open air was supplied from the air supply duct at 1 l/min. The temperature of the mixed reformed gas with air was cooled to 100 degreesC with the heat exchanger 7 in which water is circulated to cause reaction between CO and oxygen at the catalyst layer 1. The catalyst layer 1 was heated with a heater 8 at the downstream side to a temperature of 150degreesC. Measurement of temperature distribution on the catalyst layer 1 indicated an almost linear elevation from upstream side toward downstream. The composition of the reformed gas leaving the reformed gas outlet 10 after it had passed through the catalyst layer 1 was measured by gas chromatography. The result indicated a CO concentration of 5 ppm. The flow rate of water circulating inside the heat exchanger 7 was varied to alter the temperature of the reformed gas to 90 degreesC and 80 degreesC. The CO concentration in the resultant reformed gas was 6 ppm and 9 ppm, respectively. Then, the set temperature of the heater 8 was varied to adjust the temperature of the downstream side of the catalyst layer 1 to 140 degreesC and 160 degreesC. The CO concentration in the resultant reformed gas was 4 ppm and 9 ppm, respectively.