To provide a control device for reducing output fluctuation and its control method, capable of reducing output fluctuations by eliminating the fluctuations in the outputs of a wind power generator with respect to wind speed fluctuations.

The generator 3 for wind power generation, accompanied by output fluctuations, is connected with a power system 9 via a converter 4, a DC link 11, an inverter 5 and a transformer 6. This control system determines it as having no output for the output fluctuation reducing circuit, if a shaft speed is constant with a output fluctuation reducing apparatus which gives an exciting frequency 10 to the exciting converter 4 using a controller 8 so as to eliminate control deviations, between a signal 2c calculated at a point where the calculated result of a power command value 2a of the generator 3 and the output signal of an output fluctuation circuit 2b are combined.