We investigated the durability of Ni/TiO2 catalyst in CO selective methanation reaction. In the reaction test up to 72 h, trace amounts of chlorine component contained in the catalyst gradually decreased. As a result, the progress of the methanation of CO2 increased, leading to a decrease in the reaction selectivity of the methanation of CO in the title reaction. By contrast, in the early stage of the reaction in which the elimination of chlorine component was small, it was confirmed that the number of active sites of metallic Ni increased, and the CO methanation activity was improved. Furthermore, it was found that the chlorine component in the catalyst was more remarkably removed by exothermic reaction heat under the methanation reaction atmosphere as compared with the case of heat treatment under other atmosphere. Catalyst durability under the conditions simulating a daily start-up and shut-down operation that repeated a cycle of cooling and heating the temperature of the catalyst layer to room temperature was also evaluated. As a result, we revealed that a trace amount of chlorine component was removed by exposure to condensed water, leading to a decrease in the reaction selectivity of the catalyst.