A method of detecting a damage of a wind turbine blade of a wind turbine rotor including at least one wind turbine blade, the method comprising: a light input step of inputting light into a fiber-optic sensor (12A,12B,12C,12D) mounted to each wind turbine blade and having a grating portion at which a refractive index changes periodically in a longitudinal direction; a light detection step of detecting reflection light from the grating portion; an obtaining step of obtaining a wavelength fluctuation index which represents a fluctuation amount of a wavelength of the reflection light detected in the light detection step from a temporal change of the wavelength; and a detection step of detecting presence or absence of a damage of the wind turbine blade on the basis of the wavelength fluctuation index calculated in the obtaining step, characterized in that the detection step comprises detecting the presence or the absence of the damage of the wind turbine blade on the basis of: the wavelength fluctuation index taking account of at least one of a correlation between the wavelength fluctuation index of the wind turbine blade and a load index related to a load applied to the wind turbine blade, or a correlation between the wavelength fluctuation index of the wind turbine blade and a temperature index related to a temperature of the wind turbine blade, and a result of comparison between an evaluation value obtained from the wavelength fluctuation index of the wind turbine blade and a threshold that is a corrected threshold corrected on the basis of the correlation. a light input step of inputting light into a fiber-optic sensor (12A,12B,12C,12D) mounted to each wind turbine blade and having a grating portion at which a refractive index changes periodically in a longitudinal direction; a light detection step of detecting reflection light from the grating portion; an obtaining step of obtaining a wavelength fluctuation index which represents a fluctuation amount of a wavelength of the reflection light detected in the light detection step from a temporal change of the wavelength; and a detection step of detecting presence or absence of a damage of the wind turbine blade on the basis of the wavelength fluctuation index calculated in the obtaining step, characterized in that the detection step comprises detecting the presence or the absence of the damage of the wind turbine blade on the basis of: the wavelength fluctuation index taking account of at least one of a correlation between the wavelength fluctuation index of the wind turbine blade and a load index related to a load applied to the wind turbine blade, or a correlation between the wavelength fluctuation index of the wind turbine blade and a temperature index related to a temperature of the wind turbine blade, and a result of comparison between an evaluation value obtained from the wavelength fluctuation index of the wind turbine blade and a threshold that is a corrected threshold corrected on the basis of the correlation. the wavelength fluctuation index taking account of at least one of a correlation between the wavelength fluctuation index of the wind turbine blade and a load index related to a load applied to the wind turbine blade, or a correlation between the wavelength fluctuation index of the wind turbine blade and a temperature index related to a temperature of the wind turbine blade, and a result of comparison between an evaluation value obtained from the wavelength fluctuation index of the wind turbine blade and a threshold that is a corrected threshold corrected on the basis of the correlation.