To prevent disconnection of wind turbine generators due to changes in wind conditions even if the wind turbine generators is operated to limit the power output. A plurality of wind turbine generators that generate electric power by the rotation of rotors are provided in a wind farm are interconnected and are operated while the power outputs are limited in advance so as to be able to further supply electric power to a utility grid in response to a decrease in the frequency or voltage of the utility grid. The rotational speed of the rotor, which is a physical quantity related to increase and decrease in the power output of the wind turbine generator, is measured for each of the wind turbine generators by a wind-turbine control system, and the limitation amount of the power output of the wind turbine generator is set for each of the wind turbine generators by a central control system on the basis of the rotational speed of the rotor measured.