PURPOSE: To prevent the slip-off of a speed change gear by preventing the shift in the axal direction of the gear at a speed change position by the simple constitution in which the speed change gear is speed-changed by the spline fitting for a speed change shaft.

CONSTITUTION: On the spline fitting part 3 between a speed change gear 1 and a speed change shaft 2, an annular resistance groove 6 which is formed by cutting a spline stripe 4 on the inner periphery of the speed change gear 1 in the axis turning direction is formed. Further, an annular resistance groove 7 is tonaed by cutting a spline stripe 5 on the outer periphery of the speed change shaft 2 in the axis turning direction.