A method of controlling a wind turbine generator, comprising: accumulating a cumulative damage degree Du of each evaluation point of the wind turbine generator in a unit period over an entire evaluation period, to calculate a total cumulative damage degree Dt of the entire evaluation period at each evaluation point; comparing the total cumulative damage degree Dt at each of the evaluation points calculated by the calculation step with a predetermined first threshold value (P ∗ Q) and comparing an increase rate dDt/dt of the total cumulative damage degree Dt at each evaluation point with a second threshold value to evaluate fatigue of a part to which each evaluation point belongs in the wind turbine generator; and determining an operation mode of the wind turbine generator based on an evaluation result of the evaluation step into a normal operation mode when the total cumulative damage degree Dt is less than the first threshold value (P ∗ Q) and the increase rate dDt/dt is less than the second threshold value, and determining the operation mode into a low-load operation mode in which an output is suppressed as compared with the normal operation mode if the total cumulative damage degree Dt is equal to or larger than the first threshold value (P*Q) or the increase rate dDt/dt is equal to or larger than the second threshold value, wherein the step of determining the operation mode is performed repeatedly at a determination interval that varies between 1 day and 6 month, wherein after the total cumulative damage degree Dt increases above the first threshold value (P*Q) and the operation mode is changed to the low-load operation mode, the operation mode returns from the low-load operation mode to the normal operation mode when it is determined that the total cumulative damage degree Dt again is below the first threshold value (P*Q) in an upcoming operation-mode determination in which the step of determining the operation mode is performed after the determination interval. accumulating a cumulative damage degree Du of each evaluation point of the wind turbine generator in a unit period over an entire evaluation period, to calculate a total cumulative damage degree Dt of the entire evaluation period at each evaluation point; comparing the total cumulative damage degree Dt at each of the evaluation points calculated by the calculation step with a predetermined first threshold value (P ∗ Q) and comparing an increase rate dDt/dt of the total cumulative damage degree Dt at each evaluation point with a second threshold value to evaluate fatigue of a part to which each evaluation point belongs in the wind turbine generator; and determining an operation mode of the wind turbine generator based on an evaluation result of the evaluation step into a normal operation mode when the total cumulative damage degree Dt is less than the first threshold value (P ∗ Q) and the increase rate dDt/dt is less than the second threshold value, and determining the operation mode into a low-load operation mode in which an output is suppressed as compared with the normal operation mode if the total cumulative damage degree Dt is equal to or larger than the first threshold value (P*Q) or the increase rate dDt/dt is equal to or larger than the second threshold value, wherein the step of determining the operation mode is performed repeatedly at a determination interval that varies between 1 day and 6 month, wherein after the total cumulative damage degree Dt increases above the first threshold value (P*Q) and the operation mode is changed to the low-load operation mode, the operation mode returns from the low-load operation mode to the normal operation mode when it is determined that the total cumulative damage degree Dt again is below the first threshold value (P*Q) in an upcoming operation-mode determination in which the step of determining the operation mode is performed after the determination interval.