PURPOSE: To perform stepless speed change without interrupting the transmission of power from a gear to another gear of different rotating speed by providing plural gears of different rotating speed in such a way that the respective tooth faces are arranged in the axial direction, and providing gears sliding with each of these gears separately so as to be engaged by splines separate from a shaft.

CONSTITUTION: When a holding frame 16 is moved by operating a speed change rod 19, a main speed change gear 10 is going to slide with an intermediate gear 7. With further movement, a linear spline groove 14 absorbs rotational difference between both intermediate gears 8, 7 so as to be formed into spiral shape 12 corresponding to this rotational difference. On the other hand, an auxiliary speed change gear 11 goes away from the intermediate gear 8. The shape of the spiral spline groove 12 of the main speed change gear 10 is therefore shallowed (13) gradually so as to be finally formed into the linear spline groove 14. In this way, the rotation of the intermediate gear 7 can be transmitted correctly, thus enabling stepless speed change.