To efficiently control the rotating speed ratio of left-right wheels over a wide range with a simple constitution.

Left-right output shafts 10L and 10R of a differential device 2 are respectively connected to a pump shaft of left-right hydraulic pumps 3L and 3R, and are also connected to left-right axles 11L and 11R via this pump shaft. A delivery port of the left side hydraulic pump 3L and a suction port of the right side hydraulic pump 3R are communicated via a hydraulic passage 15, and a delivery port of the right side hydraulic pump 3R and a suction port of the left side hydraulic pump 3L are communicated via a hydraulic passage 16. A controller 50 sets the target rotating speed ratio of left-right wheels in traveling, and efficiently controls the rotating speed ratio of the left-right wheels over a wide range by operating the left-right hydraulic pumps 3L and 3R by a pump and a motor by varying the pump displacement volume so as to get to this target rotating speed ratio.