Fluid machinery driven by a motor and generating pressures by revolution of an impeller, which is provided with a frequency converter (F) for supplying the motor with electricity, means for detecting frequencies and current values, and a program for prescribing the relationship between frequencies and current values beforehand, and wherein the frequencies and current values in an actual operation are compared with the prescribed program so that frequencies generated by the frequency converter (F) change so as to bring an actual operation point of the machinery closer to the prescribed program.
