Yaw control is performed such that a nacelle faces into an actual main wind direction. The actual wind direction is estimated by detecting the main wind direction with an anemoscope (wind direction detecting means), assuming the actual wind direction by assuming a wind direction offset value, which is a deviation between the main wind direction and the actual wind direction, which is a direction of wind which blows in actual use, at a predetermined wind speed with a wind direction assuming unit (wind direction assuming means), calculating an average generator output power for a predetermined period of time in the assumed actual wind direction with an average-generator-output-power calculation unit (average-generator-output-power calculation means), approximating the average generator output power with respect to the assumed wind direction offset value to a quadratic curve with an actual wind direction estimation unit (actual wind direction estimation means), and estimating the wind direction offset value at the time when the average generator output power is the maximum in the approximated quadratic curve to be an actual offset value.