PURPOSE: To dispense with boring work, and disposition of a pipeline or special filter, simplify the constitution, facilitate assembling work, reduce a cost, prevent intrusion of splashing particles of magnetic fluid into a chamber due to breakage of a magnetic fluid seal unit.

CONSTITUTION: A plurality of permanent magnets 7 and pole pieces 8 are alternately arranged inside a housing 6 for inserting a power transmitting shaft 2 into a chamber 1 in the vacuum ambience from the atmosphere. Magnetic fluid 9 is held between the pole pieces 8 and the circumferential surface of the shaft 2 by a magnetic flux, thus forming a multiple-stage magnetic fluid seal unit 10. A pair of pole pieces 14 holding a permanent magnet 13 are disposed nearer to the chamber than the multiple-stage magnetic fluid seal unit 10 inside the housing 6. An empty dry magnetic gap portion 12 where the magnetic fluid is not filled is interposed between the pole pieces 14 and the circumferential surface of the shaft.