In operating the carbon monoxide removal reactor or the fuel reforming system, there is provided a technique for removing carbon monoxide in a stable manner for an extended period of time. In a method of removing carbon monoxide including an introducing step of introducing a reactant gas including mixture gas and an oxidizer added thereto to a carbon monoxide removal reactor forming in its casing a catalyst layer comprising a carbon monoxide removal catalyst for removing carbon monoxide contained in the mixture gas and a removing step of removing the carbon monoxide by causing the oxidizer to react with the mixture gas on the carbon monoxide removal catalyst, in said introducing step, the reactant gas of 100° C. or lower is introduced to the carbon monoxide removal reactor.