PURPOSE: To simplify a transmission gear in structure by constituting each transmission over an interval between an input rotor of a differential gear, either of paired differential shafts and input shaft, via two transmission gears whose one side is set down to a shift transmission belt being shift transmittable.
CONSTITUTION: If an input shaft 5 is rotated for transmission, an input rotor 2 and a differential shaft 3 of a differential gear 1 are rotated for transmission by way of transmission gears 7, 8. With each rotation of the input rotor 2 and the differential shaft 3, another differential shaft 4 is rotated for outputting. At this time, the transmission gear 8 consists of a shift transmission belt 6, and the differential shaft 4 is stopped and neutralized by differential transmission when rotation of the shaft 3 is the same as rotational frequency of the rotor 2 by speed change of the belt. If this speed change by the belt 6 from this neutral position is made into low speed in order, rotational frequency of the shaft 3 to the rotor 2 comes low so that the shaft 4 increases its speed clockwise. In contrast with this, if the speed change by the belt 6 is made into high speed in order, the rotational frequency of the shaft 3 to the rotor 2 comes high and thus rotation of the shaft 4 increases counterclockwise.