To ensure lubricity without deteriorating the strength of a differential case.

This device is provided with a differential case 3; side gears 19, 21; four pairs of housing holes 37, 39; pinion gear pairs; a first oil groove 49 formed on an inner circumference of a boss part 11 of the differential case 3; a second oil groove 51 which is formed in axial direction on an inner circumferential surface of a heavy wall thickness part of a bearing part 27 in the differential case 3 on intermediate parts of adjacent housing hole pairs, and whose outer side end leads oil in the first oil groove 49 and inner side end is communicated with a sliding surface with the side gear 19; and a third oil groove 59 whose one end is slantly crossed in a radial direction and extendedly arranged while being communicated with an inner side end of the second oil groove 51, and the other end is communicated with one side of the housing hole 39.