PURPOSE: To provide a rotary piston vacuum pump to perform inner compression having favorable inner seal, high inner compression, and the minimum ineffective space.

CONSTITUTION: In a pump housing, an action piston 4 and a control piston 5 having circular cross sectional surfaces are provided. The action piston 4 has a piston head 6, and the control piston 5 has a recessed part 7. Disposition of the action piston 6, the control piston 5, partial zones 8, 9 of the pump housing, and a gas exit opening 3 assures an effective pump to perform inner compression and minimize loss. For preventing backflow of gas, an inner hole 14 is provided in the control piston 5. Overpressure at a terminal end of a compression process is balanced by an inner hole 15 in the housing.