PURPOSE: To enable the transition between two conditions of transmission and cut-off by forming a recess in the middle part of the outer peripheral part of an outer shell internal ring gear, and forming, in the recess, timing teeth by which a stopper mechanism can stop the outer shell internal ring gear.

CONSTITUTION: There are provided a sun gear 1, a suitable number of planetary gears surrounding the sun gear, an outer shell internal gear 3 surrounding the planetary gears and meshed with the latter, and a carrier 4 for rotatably supporting planetary gears by means of planetary shafts. Further, the outer shell internal ring gear 3 has slippable outer peripheral surfaces 25, 26 which are smooth cylindrical surfaces on both sides of the outer peripheral part of the outer shell internal ring gear 3, and a recess is formed in the intermediate part of the outer peripheral part, and timing teeth 28 are formed in the recess. With this arrangement, the slippable outer peripheral surfaces 25, 26 are rotatably supported by a bearing mechanism 30 so that a stopper mechanism can stop the outer shell internal ring gear 3 by means of the timing teeth 28. With this arrangement, both power transmission and cut-off conditions can be taken, and further, the transition between both conditions can be made at an arbitrary time.