NOVELTY - A mixed gas containing hydrogen, carbon monoxide and carbon dioxide is pretreated and subjected to methanation process by circulating the mixed gas to the catalyst layer formed by filling a reaction container with specific methanation catalyst and carrying out methanation removal process. The carbon monoxide concentration of the mixed gas is reduced by methanation reaction such that the carbon monoxide concentration in the inlet port of the catalyst layer is 0.1 vol.% or less, in the removal of carbon monoxide.    USE - Removal of carbon monoxide from mixed gas supplied to solid high polymer type fuel cell (claimed).    ADVANTAGE - The method enables effective removal and reduction of concentration of carbon monoxide from mixed gas, with suppressed loss of hydrogen. The polymer electrolyte fuel cell supplied with mixed gas of reduced concentration of carbon monoxide has high operation efficiency.    DETAILED DESCRIPTION - A mixed gas containing hydrogen, carbon monoxide and carbon dioxide is pretreated and subjected to methanation process by circulating carbon monoxide in the mixed gas to the catalyst layer formed by filling a reaction container with methanation catalyst and carrying out methanation removal process. The carbon monoxide concentration of the mixed gas is reduced by methanation reaction such that the carbon monoxide concentration in the inlet port of the catalyst layer is 0.1 vol.% or less, in the removal of carbon monoxide. The methanation catalyst is the carbon monoxide removal catalyst formed by supporting component (I) containing molybdenum and component (II) containing ruthenium on alumina support, or carbon monoxide removal catalyst formed by supporting nickel to a zirconia support, used in the methanation reaction. The concentration of carbon monoxide is reduced to less than 1x 10-3 vol.%. An INDEPENDENT CLAIM is included for solid high polymer type fuel cell system, which is supplied with the mixed gas from which carbon monoxide is removed and reduced to a concentration of less than 1x 10-3 vol.%.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of the solid high polymer type fuel cell system. (Drawing includes non-English language text)    Raw fuel supply system (1)    Desulfurizer (2)    Steam generation device (3)    Modifier (4)    Carbon-monoxide-conversion device (5)    Polymer electrolyte fuel cell (8)