An object is to rapidly recover from a voltage drop in a power grid. When the value of a grid voltage detected by a grid voltage detection unit (22) has fluctuated so as to satisfy a condition set in advance for a reference value, a control device (19) controls a wind turbine generator (14) such that a reactive current corresponding to a required output current that is determined by an active current and the reactive current and that indicates an output level to a power grid (18) on the basis of the detection result from the grid voltage detection unit (22) is output to the power grid (18).