A method of measuring methanol vapor concentration on a real-time basis and a methanol vapor concentration measuring apparatus used in fuel cells. The method of measuring methanol vapor concentration involves using an absorption spectrometry technique, that is, after measuring intensities I0 and I1 of light of a laser before and after passing through a space filled with methanol vapor and into which light is irradiated, the methanol vapor concentration is calculated by substituting the intensities I0 and I1 into an equation defined as I1/I0=exp (K·L·C), where K represents the absorption coefficient, L represents the length of the space through which the laser passes, and C represents the methanol vapor concentration.