The present invention provides a method and device of quantitatively or qualitatively examining and diagnosing chronic fatigue syndrome (CFS) by: irradiating a sample derived from an examinee or other animal with light having a wavelength in a range of 400 nm to 2500 nm or a wavelength in part of the range detecting reflected light, transmitted light, or transmitted and reflected light to obtain an absorption spectral data and analyzing absorbance at all measurement wavelengths or at specific wavelengths in the absorption spectral data by using an analytical model prepared beforehand.
