A spectral analysis device includes a light source, a support body, a linear polarization filter, a detection unit, a regression computation unit, and an absorbance spectrum calculation unit. The support body is fixed such that an incident angle of the light is a predetermined incident angle θ. The linear polarization filter is configured such that lights with polarization angles ϕn ranging from 0° to 90° are irradiated to the support body. The detection unit detects a transmitted spectrum S from transmitted lights with the polarization angles ϕn. The regression computation unit obtains an in-plane spectrum sip and an out-of-plane spectrum sop through regression analysis by using the transmitted spectrum S and a mixing ratio R. The absorbance spectrum calculation unit calculates an in-plane absorbance spectrum Aip and an out-of-plane absorbance spectrum Aop of the thin film based on the in-plane spectrum and the out-of-plane spectrum.