When a low voltage event occurs in a utility grid, a load on a spindle system is repressed so that a damage is prevented. A rotor having a plurality of wind turbine blades, a generator to be driven by rotation of the rotor, and a pitch angle controller for controlling a pitch angle of the wind turbine blade according to a generator speed are provided, and when a voltage of a utility grid is a predetermined value or less, the pitch angle controller controls the pitch angle so that torsional vibration generated on a spindle system for transmitting the rotation of the rotor to a generator system is repressed.