PURPOSE: To prevent oil leakage without enhancing the processing accuracy by arranging a seal plate on one axial end surface of a pump gear movably in the axial direction and bringing one end surface of a pump gear in close contact with the inner surface of a casing as well as bringing the seal plate in sliding contact with the other end surface by means of actuating oil pressure caused by pressure rise.

CONSTITUTION: Power is transmitted between a transmission shaft and a front differential gear by means of raising the pressure of the operating oil inside an oil pressure producing chamber with a driven gear 25 and a ring gear 24 in accordance with a rotational difference between front and rear wheels. A seal plate 34 is brought in sliding contact with one end surface in the axial direction of a pump gear 25 and also movably arranged in the axial direction and the seal plate 34 is brought in sliding contact with one end surface of the pump gear 25 while the other end surface is brought in close contact with the inner surface of a casing 21 by means of actuating oil pressure caused by pressure rise on the seal plate 34. Thus oil leakage from a gap between the end surface of a driven gear and a casing can be restrained without any necessity of raising dimensional accuracy of the respective parts so much.