A scale composition determination system that determines a composition of a scale generated on a surface of a steel material, the scale composition determination system comprising: a detection means that detects spectral radiance of the steel material at each of a plurality of wavelengths; a temperature acquisition means that acquires a temperature of the steel material; a spectral emissivity deriving means that derives spectral emissivity of the steel material at each of a plurality of the wavelengths based on the temperature of the steel material acquired by the temperature acquisition means and the spectral radiance of the steel material at each of a plurality of the wavelengths, the spectral radiance detected by the detection means; and a determination means that determines whether or not hematite (Fe 2 O 3) has been generated in an outermost layer of the scale based on the spectral emissivity of the steel material at each of a plurality of the wavelengths, the spectral emissivity derived by the spectral emissivity deriving means, wherein the determination means determines that the hematite (Fe 2 O 3) has been generated in the outermost layer of the scale in the case where at least one of the spectral emissivities of the steel material at a plurality of the wavelengths is out of a predetermined range set at each of a plurality of the wavelengths, and determines that the hematite (Fe 2 O 3) has not been generated in the outermost layer of the scale in the case where all of the spectral emissivities of the steel material at a plurality of the wavelengths is within the predetermined range set at each of a plurality of the wavelengths, wherein in the predetermined range set at the wavelength, spectral emissivity of wustite (FeO) at the corresponding wavelength is included, a plurality of the wavelengths are determined by using the relationship between the spectral emissivity of the hematite at each of a plurality of the wavelengths and a thickness of the hematite within a range assumed as the thickness of the hematite, and a plurality of the wavelengths are determined to make the spectral emissivity of the hematite at at least one wavelength of a plurality of the wavelengths fall outside the predetermined range set at the corresponding wavelength at any thickness of the hematite in the relationship. a detection means that detects spectral radiance of the steel material at each of a plurality of wavelengths; a temperature acquisition means that acquires a temperature of the steel material; a spectral emissivity deriving means that derives spectral emissivity of the steel material at each of a plurality of the wavelengths based on the temperature of the steel material acquired by the temperature acquisition means and the spectral radiance of the steel material at each of a plurality of the wavelengths, the spectral radiance detected by the detection means; and a determination means that determines whether or not hematite (Fe 2 O 3) has been generated in an outermost layer of the scale based on the spectral emissivity of the steel material at each of a plurality of the wavelengths, the spectral emissivity derived by the spectral emissivity deriving means, wherein the determination means determines that the hematite (Fe 2 O 3) has been generated in the outermost layer of the scale in the case where at least one of the spectral emissivities of the steel material at a plurality of the wavelengths is out of a predetermined range set at each of a plurality of the wavelengths, and determines that the hematite (Fe 2 O 3) has not been generated in the outermost layer of the scale in the case where all of the spectral emissivities of the steel material at a plurality of the wavelengths is within the predetermined range set at each of a plurality of the wavelengths, wherein in the predetermined range set at the wavelength, spectral emissivity of wustite (FeO) at the corresponding wavelength is included, a plurality of the wavelengths are determined by using the relationship between the spectral emissivity of the hematite at each of a plurality of the wavelengths and a thickness of the hematite within a range assumed as the thickness of the hematite, and a plurality of the wavelengths are determined to make the spectral emissivity of the hematite at at least one wavelength of a plurality of the wavelengths fall outside the predetermined range set at the corresponding wavelength at any thickness of the hematite in the relationship.