PURPOSE: To provide a power transmission device by which the steering stability at the time of low-μ-road traveling can be enhanced, and further the durability of the device can be enhanced.

CONSTITUTION: A hydraulic power transmission device is that by which power transmission is carried out between a casing 29 and a retainer 30 by raising the pressure of operating oil inside hydraulic pressure generating chambers (A to H) in accordance with the difference in rotation between the casing 29 and the retainer 30 by means of a driven gear 31 and a ring gear 32. The device is provided with an axially moving means arranged on one end of the retainer 30 and consisting of steel balls 43 and a cam plate 41 by which when a difference in rotation is caused between the front wheels and the rear wheels, the retainer 30 is moved in the axial direction by the relative rotation between the casing 29 and the retainer 30, and a connecting means arranged on the other end of the retainer 30 and consisting of a male taper surface 44a and a female taper surface 47a by which the casing 29 and the retainer 30 are connected by the axial movement of the retainer 30.