A wind turbine generator comprises a nacelle for mounting a wind turbine rotor, a nacelle turning mechanism, a wind vane/anemometer, and a controller for controlling the nacelle turning mechanism. The controller calculates the deviation in direction of wind from the direction of wind measured by the wind vane/anemometer and the direction of the rotor of the wind turbine. The controller controls the nacelle turning mechanism to yaw the nacelle when any one of two conditions are effected; (1). such a state as the absolute value of the deviation in direction of wind exceeds a first threshold continues for a predetermined first duration, and (2). such a state as the absolute value of the deviation in direction of wind exceeds a second threshold larger than the first threshold continues for a second duration shorter than the predetermined first duration.
