PURPOSE: To execute a stable operation, while suppressing a water level in a surge tank in a hydroelectric power plant within a limit value.

CONSTITUTION: With respect to a controller 12a for calculating and outputting a guide vane opening command value which is not shown in the figure, so that a deviation of a water level detected value and its set value of a water tank in the upper part in a hydroelectric power plant becomes zero, a controller 12b for executing control of the guide vane so that a deviation of a water level detected value and its set value of a surge tank becomes zero, and a low value output computing element 13 for selecting a smaller one of outputs of these controllers 12a, 12b are provided, by which stable control can be executed, while considering a water level of the surge tank, as well.