PURPOSE: To prevent a high pressure fluid in an operating chamber or in a rotational center body from passing through a discharge opening to the outside without making a vane thick or enlarging the closable opening uselessly at the time of precipitating the communication start timing between the discharge opening and the closable opening in order to meet a low pressure ratio.

CONSTITUTION: A discharge opening 4 is divided plurally and formed in such a way as to have an instant of covering each of the discharge openings (4a, 4b, 4c) completely or almost completely with the inner peripheral surface 1' of a vane 1 and to cut off communication between an optional discharge opening out of the discharge opening 4 and a closable opening 7 around the instant of releasing this discharge opening from the state of being covered completely or almost completely with the inner peripheral surface 1' of the vane 1.