A Fuel cleanup system (FCU) that recovers fusion fuel (tritium and deuterium) from plasma exhaust mixture gas has been developed and demonstrated at the Tritium Process Laboratory (TPL) of Japan Atomic Energy Research Institute (JAERI). We have proposed a new closed loop FCU system built up by connecting the tubular reservoir tank, the electrolytic reactor and a palladium diffuser. In the electrolytic reactor, methane and water are converted at the same time by electrochemical reaction in gas phase oxidation and reduction to liberate hydrogen isotope as a form of elemental hydrogen. The long tubular reservoir tank that is designed to store and transfer the products gas in plug flow prevents from mixing with reactants for the successive repeat processing. With this tank, high overall decontamination factor of system can be obtained by small number of circulation. As the demonstration test, mixture gas consist of hydrogen isotopes, methane and He were processed in the closed loop FCU. The electrolytic reactor and the tubular reservoir tank worked as designed successfully, and the entire loop exhibited efficient impurity processing performance. The concentration of methane was observed to decrease sharply in every processing by the electrolytic reactor from 2.3% to less than 12ppm finally.