PURPOSE: To obtain stable speed characteristics of a water turbine, by using the deviation signal obtained by subtracting the no-load opening from a guide vane opening signal as a new guide vane opening signal, and thereby automatically correcting the change of the no-load opening caused by the water-level difference.

CONSTITUTION: When the no-load position is detected by a no-load detector 18 from the fact that the frequency has reached the rated value, a guide vane opening signal 106 is stored and set as the no-load signal. After this, a deviation signal 112 obtained by subtracting the no-load opening 112 from the guide vane opening signal 106 is supplied as a new guide vane opening signal to a speed decreasing rate controller 22. Further, since guide vanes 14 are controlled by a speed signal 116 supplied from the controller 22, stable speed characteristics of a water turbine can be obtained.