PURPOSE: To increase the operating time of an induction motor and to prolong the service life thereof by sealing the housing of the induction motor oil-tightly and then feeding oil therein in order to cool the induction motor.

CONSTITUTION: A flywheel housing 32 and a clutch housing 64 containing an induction motor 2 is sealed oil-tightly and then oil is fed to the induction motor 2 thus cooling the induction motor 2 effectively. The cooling oil is fed from an oil reservoir 51 through a supply pipe 53 to a clutch disc 41 where the oil is centrifugally dispersed to the induction motor 2. Oil supply to the clutch disc 41 is regulated according to the temperature of the induction motor 2 by controlling a regulation valve 54 through a microcomputor 62 receiving an output from a temperature sensor 61. According to the invention, operating time of the induction motor 2 can be increased.