PURPOSE: To discharge impurities easily by oil sealing between two overshooting recess grooves formed on the housing side between shaft bearings for supporting rotary shafts of a multi-stage vacuum pump and a housing neck part and the neck part.

CONSTITUTION: A three-lobe type Roots rotor 3 of a multi-stage (two-stage, for example, as illustrated) vacuum pump is fixed to a rotary shaft 4 and contained in a casing 2. A first and a second overshooting recess grooves 6, 16 are formed on the side of a housing 1 between a neck part 5 of the housing 1 installed on a side part of a casing 2 and a shaft bearing 27 for supporting a rotary shaft 4 to communicate with a first and a second drain passages 10, 18. The casing 2 and the drain passage 10 are connected 11 with each other. A first and a second overshooting materials 22, 25 are formed at corresponding parts of the rotary shaft 4 to the recess grooves 6, 16. Oil seals 14, 13 of ethylene tetrafluoride resin are engaged between the overshooting recess grooves 16, 6 and the neck part 5 of the housing 1. Cleaning liquid and impurities mixed in oil can thus be discharged out easily.