PURPOSE: To improve sealing performance by keeping magnetic fluid in a gap between the screw thread tip of a screw pump and the internal surface of its cylindrical cavity by concentrating magnetic flux on the above gap.

CONSTITUTION: A spiral screw thread 1a and a land 1b for collecting magnetic flux are formed in projecting shape on the periphery of a rotary shaft 1 made of magnetic material. Magnetic fluid is taken out of the supplying port 8 for the magnetic fluid, and transferred to an exhaust port 5, and then is separated through a separator 10 from working fluid, and fed back to the supplying port 8. The magnetic flux concentrates in a gap between the tip of the screw thread 1a and the internal surface of the cylindrical cavity 2a of an external cylinder 2, and the magnetic fluid is kept in the above gap to move as the rotary shaft 1 rotates. Consequently, leakage out of the gap can be prevented to improve volumetric efficiency.