It is an object to supply, to the utmost extent, reactive power according to a demand of a utility grid side while maintaining the power factor within a predetermined range. The wind turbine generator system includes a main controller 19. The controller 19 includes a first controller 31 performing constant-reactive-power regulation, a second controller 32 performing constant-power-factor regulation, a judging unit 34 for judging whether a present operating condition is within a predetermined operating region, and a control-switching unit 35 for switching from the first controller 31 to the second controller 32 when the first controller 31 is performing the constant-reactive-power regulation and the judging unit 34 detects a deviation from the operating region.