1. A vibrational spectrum analysis method comprising: a dataset selection step of selecting a Raman spectral dataset including Raman spectral data of a plurality of unknown mixed samples acquired under the same measurement conditions, in a plurality of Raman spectral data items having different spectral profiles acquired with respect to the plurality of unknown mixed samples including a plurality of unknown components at an unknown mixture ratio by setting different measurement conditions in each of the unknown mixed samples; a spectrum selection step of selecting a Raman spectrum acquired under the same measurement conditions as those of the Raman spectral dataset selected in the dataset selection step, in Raman spectra of a benchmark molecule to be classified which is a component intended to be classified; a calculation step of calculating PCA loadings from the Raman spectral dataset selected in the dataset selection step; a correlation calculation step of calculating a correlation between the PCA loadings calculated in the calculation step and the Raman spectrum of the benchmark molecule to be classified selected in the spectrum selection step; and a determination step of determining a PCA loading which has the highest correlation with the Raman spectrum of the benchmark molecule to be classified, in PCA loadings calculated from a plurality of Raman spectral datasets having different measurement conditions.