To facilitate engagement between an internal gear and a carrier, perform reliable shifting, reduce the generation of noise during gear shifting, and provide an increased life by a method wherein the maximum outside diameter part, except an engaging tooth, of a carrier is always fitted in the inner peripheral part of an internal gear.

When an internal gear is slid to a front in the axial direction, a tooth groove 99 in the inner peripheral surface thereof and the engaging tooth 51 of a carrier C are engaged with each other. When moved to a rear, an engaging tooth 54 on an end face and an engaging tooth 53 on the gear case 6 side are engaged with each other. The maximum outside part 90, except engaging tooth 51, of the carrier C of a second stage planetary mechanism is always fitted in the inner peripheral part of the internal gear of the second stage planetary mechanism. Further, the maximum outside diameter part 91 of the carrier D of a first stage planetary mechanism adjoining the inner peripheral part of the internal gear. Thus, the internal gear axially moved in the gear case 6 is always fitted in and held at internal carriers C and D axially moved in the gear case 6 and movable without axial inclination.