TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Component: The platinum-containing catalyst portion of co-catalyst powder is arranged at upstream side of circulation direction of gas. The platinum-containing catalyst portion of platinum-containing catalyst powder is arranged at downstream side of circulation direction of gas. The platinum-containing catalyst portion is a monolithic catalyst in which layer containing platinum-containing catalyst powder is formed on inner surface of monolith support (I). The co-catalyst portion is a monolithic catalyst in which co-catalyst layer is formed on inner surface of monolith support (II). The platinum-containing catalyst layer and co-catalyst layer are formed on inner surface of monolithic support as upper layer and lower layer, respectively. The content of platinum atom and iron atom with respect to total quantity of platinum-containing catalyst powder is 0.2-3 mass% and 0.1-1.2 mass%, respectively. The sum total content of iron atom and co-catalyst atom with respect to total quantity of platinum-containing catalyst powder is 0.1-14 mass%. The molar ratio of sum total content of co-catalyst atom and iron atom with respect to content of platinum atom in platinum-containing catalyst powder is 0.15-65. The molar ratio of iron atom with respect to platinum atom is 0.15-3.5. Preferred Property: The specific surface area of inorganic supports (I,II) is 30-250 m2/g and 10-120 m2/g, respectively. EXAMPLE - Platinum-containing catalyst powder having specific surface area of 185 m2/g measured by Brunauer Emmett Teller method, was prepared using non-baked powder of Boehmite alumina, cobalt nitrate, and platinum dinitro diamine. Co-catalyst powder having specific surface area of 43 m2/g was prepared same as that of platinum-containing catalyst powder excluding platinum dinitro diamine. The platinum-containing catalyst powder and co-catalyst powder at a mass ratio of 1:1, distilled water and alumina sol were mixed and ground for 1 hour. The obtained coating slurry (200 g/l) was coated on honeycomb support obtained from cordierite. Catalyst layer was formed on inner surface of support by drying at 120degreesC for 1 hour and baking at 450degreesC for 1 hour. Thus, carbon monoxide selective oxidation catalyst was obtained. The content of platinum atom and cobalt atom with respect to total quantity of platinum-containing catalyst powder was 1.2 mass% and 1 mass%, respectively. The molar ratio of cobalt atom with respect to platinum atom was 2.8. The catalyst had excellent carbon monoxide removal capability.