PURPOSE: To improve the controllability of a control member for a bypass port by installing a valve mechanism for keeping the pressure in the second chamber communicating to the high pressure chamber side out of the pressure acting chambers, always below a prescribed value.

CONSTITUTION: The bypass ports 22 and 23 for the communication between a suction chamber 17 and a gap chamber are installed onto a rear side block 9, and opening/closing-controlled by a control member 24. The opening angle of the bypass port 23 is determined by the balance between the sum of the force of a spring 32 and the pressure in the first chamber 271 of a pressure acting chamber and the pressure in the second chamber 272, and control is performed by a valve mechanism 40 so that the pressure in the second chamber 272 communicating to the high pressure chamber side always becomes below a prescribed value. Therefore, the starting operation of the control member from the perfect operation position to the semioperation direction can be performed speedily.