PURPOSE: To recover leaked oil through a microscopic passage, and transmit excellent torque by connecting a rotary shaft and a rotor to each other by spline coupling, and omitting a part of spline teeth of at least one of the rotary shaft and the rotor.
CONSTITUTION: Oil leaked from the first side plate side passes through a microscopic clearance 40a, and reaches a clearance 45a between a rotary shaft SF and a hole 45 formed in the first side plate 40, and is returned again to an oil tank T by a return passage 43. Oil leaked from the second side plate 50 side passes through a microscopic clearance 50a, and reaches an oil chamber 58 partitioned by the inner periphery of the second side plate 50 and the outer periphery of the rotary shaft SF. The oil in this oil chamber 58 passes exclusively through a microscopic passage JS3, and is flowed to the clearance 45a between the rotary shaft SF and the hole 45 formed in the first side plate 40, and is returned again to the oil tank T by the return passage 43.