A horizontal axis wind turbine includes: a horizontal plate disposed on upwind side of a rotational central portion of a rotor; a first anemometer disposed at an upper position of upwind side of the horizontal plate; a second anemometer disposed at a lower position of the horizontal plate; and an upflow angle measuring section for measuring an upflow angle based on a wind speed difference or a wind speed ratio obtained from wind speeds measured by the first anemometer and the second anemometer.