PURPOSE: To perform high-efficient operation of a multidisc clutch when a cam is worked by arranging a multidisc main clutch to a planetary gear mechanism on the same side as the cam means side in an axial direction.

CONSTITUTION: An annular press member 77A to press a multidisc clutch 75 in an axial direction, i.e., rightward in a diagram, is located, as shown in a diagram, to the left of a main multidisc clutch 75 for limiting a differential in a manner to be spline-coupled with a boss part 57 and movable in an axial direction. A cam ring 77B composing a cam means is arranged to the right of the press member 77A. Thus, when a cam 79 is worked, the multidisc clutch 75 is directly pressed between the cam 79 and a planetary gear mechanism 55. This constitution prevents transmission of the force of the press member 77A to the multidisc clutch 75 through a solar gear 53, planetary gears 43 and 49, and the engaging part of an internal gear 41 of a differential case 31 and presses the multidisc clutch 75 with high efficiency.