PURPOSE: To prevent leakage due to differential pressure by producing magnetic field near a vane while holding magnetic fluid in a gap between the vane and slit through magnetic force.

CONSTITUTION: A crank shaft 2 is rotated to rotate a rotary piston 1 while contacting against the partition wall to suck fluid through an inlet IN while to deliver through an outlet OUT. Here a solenoid 5 and magnetic member 6 are arranged while holding the vane 4 to produce magnetic field thus to hold magnetic fluid and to prevent leakage through a gap between the vane and slit.