PURPOSE: To prevent the oil leak from a bearing box to a pump side in any operation state by installing a vane wheel for sealing between a driving chamber and a circumferential flow compression pump stage which communicates to an exhaust port and allowing the suction port and discharge port of the vane wheel to communicate to each specific part.

CONSTITUTION: A shaft 12 is supported by a bearing 21 in a housing 22, and driven by a motor 16, and the lubricating oil 23A in an oil tank 23 is pumped up by the centrifugal force of a tapered hole 12A and jetted out from a small hole 12B and lubricates the bearing 21. In this case,a circumferential flow pump stage 15 for shaft sealing is installed, together with a centrifugal compression pump stage 13 and a circumferential flow compression pump stage 14 in the casing 11. The suction port of the pump stage 15 is connected with the housing 22 by a connecting pipe 18 having an oil filter 17 installed midway. Further, the discharge port is opened into the installation part of the bearing 21. Therefore, in any operation state, a stabilized air stream can be supplied into the shaft seal part.