A scale composition determination device (10) determines that Fe2O3 has been generated in the outermost layer of a scale (SC) in the case where at least one of spectral emissivities at one wavelength and the other wavelength that are measured by radiometers for spectral emissivity measurement (21a, 21b) is not within a predetermined range including spectral emissivities of FeO at one wavelength and the other wavelength, and determines that Fe2O3 has not been generated in the outermost layer of the scale (SC) in the case where all of the spectral emissivities at one wavelength and the other wavelength that are measured by the radiometers for spectral emissivity measurement (21a, 21b) is within the predetermined range including the spectral emissivities of FeO at one wavelength and the other wavelength.