A catalyst for selectively oxidizing carbon monoxide in reformed gas by oxygen gas is disclosed. The catalyst for selective oxidation of carbon monoxide comprises ruthenium and/or platinum carried by a porous carrier, this porous carrier contains .alpha.-alumna, and purity of this alumna is not less than 99.95%.A particle diameter of the ruthenium and/or platinum is not larger than 200 .ANG..According to this catalyst for selective oxidation of carbon monoxide, it is possible to provide the catalyst, which reduces the carbon monoxide in the reformed gas by oxidizing it selectively, and possible to realize excellent efficiency of fuel utilization and favorable generation efficiency.