PURPOSE: To prevent heat-generation of a friction plate and to eliminate the deterioration or seisure of lubricating oil by providing a hub member, which is arranged inside a casing, with both an oil sump, which accommodates lubricating oil, and an oil passage, which introduces the lubricating oil into a friction plate.

CONSTITUTION: When oil pressure is supplied to actuators 133 and 135, piston members 109 and 111 go forward to push multiple-disc clutches 89 and 91, and consequently the driving force is transmitted to right and left rear wheels 25 and 27. Then, the lubricating oil enclosed in a differential carrier 35 flows through ports 129, 131, 132, 134, 136, etc. of a differential casing 29 or the like into an oil sump 61 to be accommodated. As hub members 45 and 47 rotate, the lubricating oil is supplied via oil passages 67 and 69 to friction plates 83, 85, and 87 to lubricate and cool the plates.