The catalytic oxidation and adsorption is the most conventional and reliable method for removing tritium that is accidentally released into the working area of fusion power plants. The authors conducted a study on the effect of water on the oxidation reaction over a Pt/MS5A catalyst, and it was found that coexistent water inhibit its catalytic activity. With this result, a simulation study was carried out, and its result suggests that the installation of a pre-adsorption bed is very effective.