PURPOSE: To prevent the generation of malfunction attributable to the reduction of the volume of the working fluid by suppressing the reverse flow of the working fluid from a working fluid keeping space to a working fluid inlet even when the opening of the working fluid inlet between a rotor and a stator is increased in size.

CONSTITUTION: A ring member 30 projected from a clearance part 25 on the inner diameter side between a rotor 18 and a stator 19 to a working fluid keeping space 20 between the rotor 18 and the stator 19 is fixed in the vicinity of a working fluid inlet 24 on the stator 19 which is arranged opposite to the rotor 18.