AbstractAs a vacuum pumping system for a D—T fusion reactor, a closed type of pumping system which does not exhaust DT fuel gas and removes only impurity gas including helium ash is proposed. The closed system is defined as a closed vacuum circuit in which only DT fuel gas can be circulated and refueled to the reactor. The basic components in the system consist of turbomolecular pumps, dry vacuum pumps, cryocondensation pumps and gas reservoirs. Helium ash removal is affected by use of getter materials for DT gas. Other impurity gas is pumped with 50 K cryocondensation pumps, and at the regeneration phase the pumped gas is dealt with by a tritium clean-up system. The detailed design of the system is described and the possibility of the system for practical use is discussed.