PURPOSE: To continue the stable power generation operation and improve the reliability of a power generation facility by providing a ring gate adjusting the inflow quantity of water into a water turbine in conjunction with a float, and restricting the inflow quantity to keep the operation speed of the water turbine and a power generator within a high-efficiency range.

CONSTITUTION: When the inflow quantity from a conducting pipe 1 to a water tank 2 becomes a preset value or above, a float 8 is lifted to lift a ring gate 10, the inflow port of a water turbine 3 is fully opened, the phase of the magnetic field is controlled by a variable-speed power generation control device 6, and the electric power is fed to a power transmission line 7. When the inflow quantity into the water tank 2 is decreased, the water level is lowered, the rotating speed of the water turbine 3 is decreased within the adjustment range, and the load of a power generator 5 is decreased at the same time. The float 8 is lowered, the interlocked ring gate 10 is also lowered, the inflow quantity of water into the water turbine 3 is restricted, the outflow quantity to a drainage pipe 4 is decreased to keep the water level, and the water pressure fluctuation is reduced. When the inflow quantity into the water tank 2 is increased, the float 8 is lifted, the interlocked ring gate 10 is lifted, and the inflow port of the water turbine 3 is returned to the fully-opened state.