An object of the present invention is to, if a frequency decrease has occurred in a power system, avoid a trip in a wind turbine generator and reliably contribute to recovery of the frequency of the power system. The present invention provides a wind turbine generator that, if a predetermined frequency decrease that requires PFR has occurred in the power system, calculates a marginal active power based on the difference between a predetermined rotational speed lower limit value, which is obtained by providing the parallel-off rotational speed with a predetermined margin, and a measured value of the shaft rotational speed, and generates an active power command value based on the marginal active power.