A spectroscopic analysis device, comprising: a detector configured to detect measurement light obtained by irradiating, with irradiation light, a sample disposed on a film on which surface plasmons are generated, wherein the sample contains at least one substance, the measurement light comprises information on an optical spectrum of the sample, and the information comprises a resonance spectrum of the surface plasmons and an absorption spectrum of the sample; and a processor configured to: calculate a peak wavelength of the surface plasmon resonance in a wavelength band in which the resonance spectrum and the absorption spectrum are generated; calculate a peak absorbance of the at least one substance based on an absorption band of the at least one substance; derive the refractive index of the sample corresponding to the peak wavelength of the surface plasmon resonance; and calculate a percentage of the at least one substance contained in the sample based on the refractive index of the sample and the peak absorbance of the at least one substance, wherein the absorption peak of the surface plasmon resonance and the absorption band of the at least one substance do not coincide, and wherein the processor is further configured to: calculate a first percentage of the at least one substance contained in the sample based on the peak wavelength of the surface plasmon resonance; calculate a second percentage of the at least one substance contained in the sample based on the peak absorbance of the at least one substance; and output the percentage of the at least one substance contained in the sample when the first percentage and the second percentage match within a predetermined margin of error. a detector configured to detect measurement light obtained by irradiating, with irradiation light, a sample disposed on a film on which surface plasmons are generated, wherein the sample contains at least one substance, the measurement light comprises information on an optical spectrum of the sample, and the information comprises a resonance spectrum of the surface plasmons and an absorption spectrum of the sample; and a processor configured to: calculate a peak wavelength of the surface plasmon resonance in a wavelength band in which the resonance spectrum and the absorption spectrum are generated; calculate a peak absorbance of the at least one substance based on an absorption band of the at least one substance; derive the refractive index of the sample corresponding to the peak wavelength of the surface plasmon resonance; and calculate a percentage of the at least one substance contained in the sample based on the refractive index of the sample and the peak absorbance of the at least one substance, calculate a peak wavelength of the surface plasmon resonance in a wavelength band in which the resonance spectrum and the absorption spectrum are generated; calculate a peak absorbance of the at least one substance based on an absorption band of the at least one substance; derive the refractive index of the sample corresponding to the peak wavelength of the surface plasmon resonance; and calculate a percentage of the at least one substance contained in the sample based on the refractive index of the sample and the peak absorbance of the at least one substance, wherein the absorption peak of the surface plasmon resonance and the absorption band of the at least one substance do not coincide, and wherein the processor is further configured to: calculate a first percentage of the at least one substance contained in the sample based on the peak wavelength of the surface plasmon resonance; calculate a second percentage of the at least one substance contained in the sample based on the peak absorbance of the at least one substance; and output the percentage of the at least one substance contained in the sample when the first percentage and the second percentage match within a predetermined margin of error. calculate a first percentage of the at least one substance contained in the sample based on the peak wavelength of the surface plasmon resonance; calculate a second percentage of the at least one substance contained in the sample based on the peak absorbance of the at least one substance; and output the percentage of the at least one substance contained in the sample when the first percentage and the second percentage match within a predetermined margin of error.