The present invention provides a method for producing a catalyst for use in preferential carbon monoxide oxidation, which catalyst has a high preferential carbon monoxide oxidation activity and a high methanation activity with respect to the carbon monoxide contained in hydrogen gas, can thus stably reduce the carbon monoxide concentration to an extremely lower level and comprises porous inorganic oxide support particles and, on the basis of the mass thereof, 0.01 to 10 percent by mass of ruthenium and 0.01 to 1 percent by mass of platinum, loaded on the support. The method comprises (1) a step of loading 30 to 70 percent of the total amount of ruthenium to be loaded, on the support particles by a competitive adsorption method and (2) a step of loading the rest of the total amount of ruthenium to be loaded and the total amount of platinum to be loaded, on the ruthenium-loaded support particles produced in step (1) without using a competitive adsorption agent.