PURPOSE: To eliminate waste time liable under exchange of gear and enable stageless change of the swirling radius under swirling operation by providing a means to couple the input and output each other with two stageless speed change units.

CONSTITUTION: Transmission gears 5, 7 and 6, 8 are fixed on the rotation axes of the stageless speed change units 26, 27, respectively, and set in engagement with transmission gears 4, 9 and 4, 12, 13, respectively. This gear 9 is fixed to the transmission shaft 37 and the gear 9 is fixed to the transmission shaft 35 while the gear 4 is mounted with possibility to rotate. On the transmission shaft 28, the transmission gears 3, 10, 13 to be engaged with the transmission gears 2, 11, 14 shall be mounted with possibility to rotate. Further on the shaft 31 of the differential device 29, the sun gears 16a, 17a to be coupled with the gears 11, 14 shall be mounted with possibility to rotate, and the gear 16a and the gear 17a are set in engagement with the planet gears 32, 33, respectively. The input gear 1 is fixed to the input shaft 30, on which the gear 2 is fixed, and the output is taken out from the output shafts 24, 25.