PURPOSE: To assure a gap throughout the rapid rotation of rotors, keep the clearance between the wall surface of a casing and each rotor slight and heighten volumetric efficiency, by supporting a rotor shaft by push members which are screw- engaging in the screw grooves of the bearing parts of the casing and can be adjusted.

CONSTITUTION: The shaft 5 of an elliptic gear rotor 3 is fitted with a collar 12 outside a bearing at the side of a driving shaft 11. A clearance 16 is maintained by adjustment with a push member 14 which is screw-engaged in the screw groove of a casing 1 and fitted on the shaft 5. Another push member 17 is screw-engaged in another screw groove 13 at a bearing-supported part at the opposite side and adjusted to maintain a clearance 21. Both the ends of the bearing-supported portions of the rotary shaft 6 of a driven gear rotor 4 are supported by adjustable push members 27, 28 fitted in screw grooves 25, 26. The push member 14 is provided with a seal 24. The push members 17, 27, 28 are provided with seals 22. This result in minimizing mechanical wear and internal leakage and heightening the efficiency of a pump.