PURPOSE: To easily and exactly perform the switching of input by using a system in which an input shaft and an output shaft are connected to two sun gears of a differential epicyclic gear device in which a rotary frame is crosswise and rotatably attached to a casing, and a spair input shaft is connected to the rotary frame in an interlocking manner.

CONSTITUTION: When positioning a switch lever 26 to neutral, an intermediate gear 22 is locked at a fixed shaft 30 and a transmission gear 6 is also locked likewise. As a result, the rotary frame 3 of a differential epicyclic gear device 2 is fixed. The turning force of an input shaft 7 connected with a motor with a brake is transmitted to an output shaft 8 through a bevel gear 9, epicyclic gears 12 and 13, and a bevel gear 10. At the time of service interruption, the input shaft 7 is locked and an input shaft 14 is turned by engine. When the switch lever 26 is titled to the right, a lifter 16 is slided to the left side, and a clutch body 18 is connected to the input shaft 14, the rotary frame 3 is turned by the turning force of the input shaft 14. By this, the output shaft 8 is speeded up and turned through the epicyclic gears 12 and 13 and the bevel gear 10.