PURPOSE: To prevent counterflow of the oil which has filled the rotor chamber and thereby facilitate restarting, by such an arrangement that a disc is put in pressure contact with the end face of a bearing part for the shaft of rotor, when the revolving rotor is going to stop, in order to block an opening provided in the oil supply duct.

CONSTITUTION: When the revolving rotor 2 is going to stop, the function to lift a disc 7 above the end face 9' of a bearing part for the shaft of rotor will become more active, and the disc 7 is put in pressure contact with the end face 9' by a coil spring 8 to block an opening provided in the oil supply duct 11. Consequently, no more oil will be supplied to the rotor chamber, and the oil filling this chamber is also driven out until the rotor 2 stops, so that there is no risk of oil's filling the rotor chamber which might cause a counterflow. Accordingly there is no such inconvenience any more as making impossible to restart because of the rotor chamber's being filled with oil.