A spectrum analysis method is provided in which a composition distribution of a sample in the depth direction is analyzed based on an energy spectrum of scattered particles scattered at the sample by irradiation with ion beams, the sample being composed of a single layer or multiple layers. In the spectrum analysis, the thickness of an object layer of the sample is obtained by the steps of measuring an energy spectrum of scattered particles scattered in a scattering angle (specific scattering angle) direction at which an energy spectrum corresponding to the object layer independently appears, extracting an independent energy spectrum of the object layer which independently appears in the measured energy spectrum, and calculating the thickness of the object layer based on a spectrum area surrounded by the waveform of the independent energy spectrum thus extracted. Since the thickness of the object layer is analyzed based on the spectrum area instead of using the half value width of the spectrum, the analysis processing can be rapidly performed and the accuracy of the analysis can be improved.