NOVELTY - The method comprises irradiating light from a laser (20) into a space filled with methanol vapor, measuring intensities of light of the laser before (Io) and after (I1) passing through the space respectively, and calculating the methanol vapor concentration by substituting the intensities Io and I1 into an desired equation to obtain an absorption coefficient (K) of 0.005335 torr1 cm1. The light of the laser has a wavelength of 3.39 mu m.    USE - The method is useful for measuring a methanol vapor concentration of a fuel cell (claimed), where the methanol is useful as a raw material for manufacturing compounds such as synthetic dyes, medical supplies or perfumes, and as a fuel for generating electricity from a direct methanol fuel cell.    ADVANTAGE - The method is capable of rapidly, stably and uniformly measuring a methanol vapor concentration in real-time by maintaining the constant voltage (0.30-0.35V) and equilibrium state of fuel consumption, and avoids the excessive supply or an insufficient supply of fuel to the fuel cell thus inducing the stable power generation.    DETAILED DESCRIPTION - The method comprises irradiating light from a laser (20) into a space filled with methanol vapor, measuring intensities of light of the laser before (Io) and after (I1) passing through the space respectively, and calculating the methanol vapor concentration by substituting the intensities Io and I1 into an desired equation to obtain an absorption coefficient (K) of 0.005335 torr1 cm1. The light of the laser has a wavelength of 3.39 mu m. The desired equation is represented by I1/Io=exp (KLC), where L represents the length of the space through which the laser passes and C represents the measured methanol vapor concentration. INDEPENDENT CLAIMS are included for:    (1) a method for controlling methanol vapor concentration of a fuel cell;    (2) an apparatus for measuring methanol vapor concentration; and    (3) a fuel cell.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of an apparatus for measuring methanol vapor concentration.    Laser (20)    Methanol injection unit (30)    Computing unit (50)    Vacuum-maintaining unit (60)    Beam splitter. (70)