PURPOSE: To make a device compact and increase a clucth torque by providing cams for engaging a frictional clutch by a cam force and positioning the cams in a pilot clutch in radial direction and inside the pilot clutch in axial direction.

CONSTITUTION: A cam 95 is composed of cam surfaces 93, 93 formed between a carrier 53 and a cam ring 75 and a ball 91 between these surfaces. When a pilot clutch 77 is engaged. a differential torque from a differential mechanism 45 is applied to the cams 95 to produce thrust forces 101 and 103 in right and left directions. A main clutch 63 is pressed against a differential case 25 to engage by a thrust force 101 through carriers 53 and 54 and shims 65. In this way, an engaging force of the main clutch 63 is added to that of the pilot clutch 77 to increase a differential limiting force of the differential mechanism 45. As for the thrust force 103, also a differential limiting force is strengthened in the same way as mentioned before. Since the cams 95 are positioned in the pilot clutch 77 in radial direction and inside the pilot clutch 77 in axial direction, the space can be saved and the device can be made compact.