To fill gas having a sufficient rate into a displacement chamber without narrowing a cross section of a throttle part by arranging a gas filling mechanism for filling as into an intermediate vacuum chamber, and arranging the other gas filling mechanism for filling gas from the intermediate vacuum chamber into the displacement chamber.

A two stage type displacement pump is provided with two pump stages 1, 2, and an intake valve 3 and a discharge valve 4 are mounted on the first pump stage 1 in those pump stages 1, 2. Two pump stages 1, 2 are connected to each other through an intermediate vacuum chamber 5. A first gas filling mechanism 6 is arranged for filling gas into the intermediate vacuum chamber 5, and is provided with a throttle part 9. A second gas filling mechanism 7 is arranged for filling gas from the intermediate vacuum chamber 5 into a displacement chamber 8, and is provided with a throttle part 10. As a gas ballast system can be efficiently formed and a rate for filling gas into the intermediate vacuum chamber 5 can be appropriately set in the throttle part, it is possible to prevent a wrong effect from exerting on a pump characteristic.