PURPOSE: To provide the rotational speed of an output shaft without speed fluctuation relative to an input shaft by providing a pair of sliders perpendicular to each other to connect a gear driven by each slider to the single output shaft through a differential gear.

CONSTITUTION: When a crank pin 3 is rotated by the rotation of an input shaft A, sliders 4, 5 disposed movably perpendicularly to each other relative to the input shaft A through rods 6, 7 are reciprocated so that free wheels 12, 13 and 14, 15 having one-way clutches respectively rotate gears 16, 17 by rack teeth 8, 9 and 10, 11 provided on both sides of respective sliders to generate rotation from an output shaft D through a differential gear 18. When a second disk 2 is moved to and away from a first disk 1 connected to the input shaft A, the radius of a crank pin 3 is changed by slant grooves 22, 24 so that the rotational ratio of output shaft D to input shaft A is changed. Thus, the speed change operation is facilitated, and the output rotation without speed fluctuation relative to the input shaft is obtained.