To conduct suppression operation before separation occurs by providing a sensor measuring a change in predetermined physical quantity in a place where fluid flows, detecting an unstable wave prior to the separation of fluid flow based on the results of the measurement, and determining the sign of separation, thereby reducing oscillation and noise caused by the separation effectively.

A single-stage centrifugal compressor, having a diffuser attached with an impeller and a guide vane 10 in a casing, raises fluid pressure, guiding the fluid from the impeller by means of the guide vane 10. Pressure holes 12 are formed at four positions a prescribed dimension distant from the front end of the guide vane 10 and is connected to a capacitor microphone 16 through a conduit 14, so that pressure fluctuations at four positions can be obtained simultaneously. If the operation is conducted under such a condition that suction flow falls short of designed flow rate, it is determined that a sign of separation phenomenon appears in the guide vane 10 and, for example, the angle of the guide vane 10 is adjusted to approach the angle of the flow running out of the impeller.