PURPOSE: To reliably prevent the occurrence of jump-over during sudden control of transmission, by providing a stopper which controls further forward movement in a way that the outer end of a latchet is brought into elastic contact with the inner surface of a driven ring, and a transmission lever is brought into collision with the forward end of the latchet.

CONSTITUTION: When a driven ring 6 is rotated at an excessive speed by sudden stepping on a transmission pedal, even if a latchet 8 is stopped in a given position, a ring 6 and a drum cam 3 are further rotated away from the latchet 8 by means of inertia, and are tried to jump over a lock position formed with a roller 12 and a grooved pulley 10. When return motion of control of pedalling is slowly effected, after the ring 6 is rotated by the width of an engaging groove 5, a side wall is collided with the back of the latchet 8 to stop rotation. Even when return motion of control of pedalling is rapid and a support member 9 is brought into a return motion state before jump-over preventing motion by the latchet 8 is completed, if the side wall of the ring 6 is collided with the back of the latchet 8, only the latchet 8 is rotated counterclockwise against an engaging spring 14, and is brought into a stopper 4b of a transmission lever 4 to prevent the occurrence of jump-over.