To improve the compactness and weight reduction of a transmission by disposing right and left continuously variable hydrostatic transmission (hereinafter, referred to as 'HST') having both a pump and a motor of variable displacement type, driving right and left axles at the same number of revolution to advance a vehicle straight and give the difference in number of revolution to the right and left axles to turn the vehicle.

Two variable displacement pumps 101 and 201 are disposed across one hydraulic passage plate 15 so as to be driven by a same shaft 3, two variable displacement motors 151 (one of them is shown outside the figure) are disposed continuously on one side of the hydraulic passage plate 15, and each pump and each motor are stored in individual cases 104, 154 and 204.