In a controller for a windfarm including a plurality of wind turbine generators controllable in rotation frequency and pith of blades of the wind turbine generators, coupled to a grid through a transmission line, an anemometer is provided to each wind turbine generator to transmits wind direction and wind speed data to a central controller which process the wind direction and wind speed data to transmit an output command each wind turbine generator. The central controller predicts, on the basis of the wind speed detected by the upstream wind turbine generator, variation in wind speed at other wind turbine generators to control the output of the windfarm. The output power may be limited or charging energy as rotation energy of blades and discharging the rotation energy as output power in addition to a battery unit for averaging.