It is an object to stabilize a utility grid even when an unexpected fluctuation in the frequency of the utility grid occurs. A wind turbine generator includes a rotor that rotates by wind power, a generator that is driven by rotation of the rotor, and a control device that controls a power output of the generator to increase while a rotational speed of the generator decreases when a frequency of a utility grid becomes smaller than or equal to a predetermined rated frequency and when the rotational speed of the generator is greater than or equal to a first predetermined value. In this way, even when the frequency of the utility grid fluctuates, the frequency fluctuation can be suppressed, and the utility grid can be stabilized.