PURPOSE: To improve cooling efficiency in the whole operation range and to prevent excessive increase in output torque in a high rotation zone by properly increasing a circulation quantity of working fluid over the whole range from a low rotation zone to a high rotation zone for a rotor.

CONSTITUTION: A circulation promoting hole 18c letting a part of working fluid, which is fed to a working fluid holding space 20 between a rotor 18 and a stator 19, flow outward is provided in a shell 18a in the rotor 18.