PURPOSE: To prevent the vibration of an impeller of a centrifugal compressor caused by the opening/closing angle of a suction vane of a displacement control device.

CONSTITUTION: Curved fins 6 which are formed at the radius of curvature around the center in the radial direction of a suction side casing 3 are provided in a projecting manner on both sides of a suction vane 1 which is internally provided on the suction side casing 3 on the suction side of a discharge side casing 4. Then, the whirl flow of the gas to be caused by the curved fins 6 according to the opening/closing angle of the suction vane 1 is guided, and the whirling center of the whirl flow is not deviated from the axis passing through the center in the radial direction of the suction side casing 3 irrespective of the opening/closing angle of the suction vane 1, and the unbalance in the pressure of the gas to be sucked is eliminated, and an impeller 5 of a centrifugal compressor is not vibrated.