PURPOSE: To aim at elimination by a simple and cheap method of influence on an excellent torque transfer characteristics by the temperature by providing, on the vane of a hydro-coupling unit, a ring shape member composed of a form memory material having an orifice whose diameter becomes small or large according to high or low temperature.

CONSTITUTION: A through hole 19 is provided on a vance 11 for forming a hydrocoupling unit and a ring shape member 20 made of a shape memory material is fitted in the through hole 19. An orifice 18 is provided in the center hole of the ring shape member to be formed so that the orifice diameter becomes small when the temperature is high and large when the temperature is low. Consequently, the viscous resistance of the hydraulic oil passing through the orifice 18 can be kept constant even if the viscosity of the hydraulic oil varies due to its temperature variation and can be made to have an ideal toque transfer characteristics constantly under both low temperature and high temperature by a simple construction.