PURPOSE: To reduce the working man-hours for a spline which is engaged with a sleeve gear and reduce the manufacturing cost by realizing the locked condition where a sun gear is directly connected to an internal gear through the sleeve gear without forming the spline for engagement of the sleeve gear apart from the gear part formed in the respective component members of the planerary gear mechanism.

CONSTITUTION: The region where a sleeve gear 26 is engaged is made at the extension part 24a of a sun gear 24 and the extension part 14a of an external planerary gear 14 without forming the region as a separate spline apart from the gear part of the component members of the planetary gear mechanism.