To restrain an increase in rotational speed, upon the interruption of a load, and properly maintain the strength and safety of a rotating part by adjusting the closing speed of a guide vane so as to correspond to a condition for water pressure and a load in a casing.

A speed setting unit 10 performs a subtraction process regarding an output signal and a signal detected with a speed detector, and a PID commuting element 14 receives a signal for a deviation from the output signal p of a steady state speed drop rate outputting element 18, thereby outputting a proper servo motor opening degree instruction value. Also. water pressure in a casing downstream of an inlet valve is detected and the value of the water pressure is inputted to an output control characteristic computing element 40, together with an output from an opening degree detector 28. Thereafter, output calculated with the output control characteristic computing element 40 is inputted to an opening degree adjusting element 20. At the same time, adjustment is made so that the closing speed of the guide vane is kept high when a measured water pressure value is lower than a maximum water pressure value at the high pressure side. Thus, an increase in the rotational speed of a hydraulic turbine can be restrained at a low level.