PURPOSE: To reduce the frictional sound and abrasion of contacting parts, by enabling the direction of inclination of herical teeth to be specified by the positions of herical gears and engaging clutches.

CONSTITUTION: The herical teeth of right-hand screw inclination RH are provided to the idle gear 9 on a driving shaft 1, and those of left-hand screw inclination LH are provided to the idle gear 10, and further the herical teeth of right- hand screw inclination RH are provided to the idle gears 31, 32 on a driven shaft 20, and those of left-hand screw inclination LH are provided to the idle gear 32. Thus, since the direction of inclination of herical teeth can be specified by the positions of herical gears and engaging clutches 14, 37, 42, so as to eliminate the thrust added to the clutches, the friction at the contacting parts between the ring grooves of clutches and shift forks can be reduced. Accordingly, the frictional sound and abrasion of contacting parts can be reduced, and the necessity for using an antifriction material to the contacting parts can be eliminated.