PURPOSE: To stop a power transmission at a detent stop position without fail by forming a lacking tooth area so as to make both driving and driven sector gears engaged with each other just before the engagement between both driving and driven main gears is released, and thereby decelerating the driven gear short of the detent stop position.

CONSTITUTION: A driven gear 12 is in a state of being just before a detent stop position, at this time, a meshing relationship between both main gears 14 and 20 is released, and simultaneously sector gears 16, 22-1 are meshed with each other, thus deceleration of the driven gear 12 takes place. Here, as in a point A50, teeth of these sector gears are set up so as to be meshed with each other before a last tooth #9 of the main gear separates, as in a point B60. With this constitution, such a possibility that the driven gear 12 might precede in two gears' meshing relations and a bound is caused at time of gear changing is prevented from occurring. Even in the case of a partial gear 22-2, it is the same manner. Accordingly, the driven gear 12 is decelerated short of the detent stop position, and even when it is rated at high speed, this gear 12 is surely stoppable at the detent position.