A method of controlling a wind turbine generator including a rotor having at least one blade, the method comprising a control-mode switch step (S2, S9) of switching a control mode of the wind turbine generator between a first control mode in which a torque of the rotor is controlled to control a rotation speed of the rotor to be a predetermined rotation speed, and a second control mode in which a pitch angle of the blade is controlled to maintain the rotation speed of the rotor at a constant speed, wherein, the control-mode switch step (S2, S9) includes: if a first condition including a condition that a torque command value of the rotor is greater than a first torque threshold value is satisfied, switching the control mode from the first control mode to the second control mode, maintaining the torque command value of the time when the control mode is switched until a pitch-angle command value of the blade becomes smaller than a second pitch-angle threshold value, and increasing the torque command value to a maximum value and fixing the torque command value at the maximum value when the pitch-angle command value has become smaller than the second pitch-angle threshold value; and if a second condition including a condition that the pitch-angle command value of the blade is greater than a first pitch-angle threshold value is satisfied, switching the control mode from the second control mode to the first control mode, maintaining the pitch-angle command value of the time when the control mode is switched until the torque command value becomes smaller than a second torque threshold value, and increasing the pitch-angle command value to a maximum value and fixing the pitch-angle command value at the maximum value when the torque command value has become smaller than the second torque threshold value, wherein the second pitch-angle threshold value is smaller than the first pitch-angle threshold value, and the second torque threshold value is smaller than the first torque threshold value. if a first condition including a condition that a torque command value of the rotor is greater than a first torque threshold value is satisfied, switching the control mode from the first control mode to the second control mode, maintaining the torque command value of the time when the control mode is switched until a pitch-angle command value of the blade becomes smaller than a second pitch-angle threshold value, and increasing the torque command value to a maximum value and fixing the torque command value at the maximum value when the pitch-angle command value has become smaller than the second pitch-angle threshold value; and if a second condition including a condition that the pitch-angle command value of the blade is greater than a first pitch-angle threshold value is satisfied, switching the control mode from the second control mode to the first control mode, maintaining the pitch-angle command value of the time when the control mode is switched until the torque command value becomes smaller than a second torque threshold value, and increasing the pitch-angle command value to a maximum value and fixing the pitch-angle command value at the maximum value when the torque command value has become smaller than the second torque threshold value,