PURPOSE: To avoid abnormal increase in discharge pressure for aiming at preventing failure, by opening a by-pass passage communicating between an operating chamber and a suction chamber when the discharge pressure exceeds a predetermined value so that a part of fluid in the operating chamber is returned to the suction chamber for decreasing the discharge volume.

CONSTITUTION: When a condition is established such that the rotational speed of an engine is increased, and discharge pressure in a discharge chamber 6 is raised to a value exceeding a predetermined value which is set by the force of a set spring 14, a spool 13 is moved to the left as shown in the drawing, overcoming the force of the spring 14, and therefore, a circumferential groove 13a aligns with an opening 13b. As a result, an operating chamber 8 and a suction chamber 4 are communicated together through a by-pass passage 10, and therefore, a part of coolant gas sucked through a suction port 5 is returned from the opening 10b to the suction chamber 4 through the by-pass passage 10 while a vane 3 moves to the opening 10b, thereby the substantial discharge volume is decreased. With this arrangement, excessive rise in discharge pressure is avoided for preventing failures in the compressor.