PURPOSE: To return oil to a pump side without using a particular oil separator by forcing oil supplied to an upper part bearing, by providing a passage connected to a supply oil pump to feed oil to the upper part bearing and also a helical groove for forcedly returning oil, after lubricating the bearing, to a rotor of a motor.

CONSTITUTION: A supply oil pump 8 provided in a lower part is actuated according to rotating a drive shaft, to supply lube oil from this pump 8 to an upper part bearing 15 of an upper part bearing housing 14 via a communication path 17 and a forcedly supplying oil passage 18. After lubrication, oil discharged to a downward side from the bearing 15 is sprung outward in a diametric direction of the drive shaft by rotating a balance weight 34 provided in a rotor 31, to collide against a tapered surface 37a of a coil end 37, and the oil is guided to a downward side to flow in a helical groove 19 of the rotor 31 from the upper end part of a stator 32, and returned to a bottom part oil reservoir 1c from a return oil receiving chamber 52 and a return oil passage 53 under the rotor 31 by screw pump action of the helical groove 19. In this way, oil after lubrication can be returned to a pump side without providing an oil separator of demister or the like.