To prevent inundation in a hydraulic power plant so as to protect a power generating facility by monitoring a rate of water which flows in the power plant using an inundation judging element, and judging on the basis of its monitor information whether there is a possibility of the inundation or not by a real value judgment and a premeasuring value judgment, and starting control for preventing the inundation in the power plant in the case where there is the possibility of the inundation.

When it rains in torrents upstream of a position where a power generating plant building 8 is constructed, its real rain rate or its change ratio are detected by a rain gauge 21. An increasing rate of water which flows in the power generating plant is predicted by an inundation preventing controller 23, an inlet valve is closed after a power generator 5 and a hydraulic turbine 4 are stopped, a tailrace gate 24 is closed, and thereby, it is possible to shut-off that water flows into the building 8. A drain pump 15 is operated, and a water level of a drain pit is dropped to a lower limit so as to ensure a drain capacity. A drain pressurizing pump 26 is operated, and thereby, drain function is increased so as to increase drain function in the building 8.