PURPOSE: To dampproof a generator by forming an air curtain between a hydraulic turbine and the generator while the hydraulic turbine and the generator stop.

CONSTITUTION: Outside air is intaken from a duct 5 and energized by an air blower 7 and blown out from a flat air blower outlet 6. Thereafter, when an air current which becomes flat reaches the opposite side, it is bent by a curtain guide 9 and enters the side of a hydraulic turbine 3 and the air currents on the side of the generator 2 and the side of the hydraulic turbine 3 are separated from each other. Thereafter, a control device 8 starts the air blower 7 by receiving a stop signal of a hydraulic turbine generator 1, stops the air blower 7 by receiving a start signal of the hydraulic turbine generator 1 and forms an air curtain 4 only while the hydraulic turbine generator 1 stops. Consequently, it is possible to prevent moisture absorption of a generator coil without using an electric heater consuming much electric power.