PURPOSE: To reduce the sliding resistance of rotors by providing a movable side plate making contact with at least one end face of a drive and/or driven rotor, and utilizing discharge pressure to separate a side plate from the end face of the rotor.

CONSTITUTION: When a drive 15 and driven 16 rotor are rotated by rotation of a drive shaft 17, oil sucked from a suction port 12 is fed into a pump space 24 and is discharged from a discharge port 13. In this case, when the pump discharge pressure is low, the hydraulic pressure of a hydraulic room 23 communicated with the discharge port 13 is also low, so a side plate 20 is forced into sliding contact with the driven rotor 16 by the force of a spring 22. When the number of rotation of the drive shaft 17 is increased and the hydraulic pressure of the discharge port 13 is heightened, however, the side plate 20 integrated with the driven shaft 18 is moved upward against the spring 22 by the pressure of the hydraulic room 23 and the interval B between the side plate 20 and the driven rotor 16 is slightly widened, so that the sliding resistance is decreased.