A method for quantitating a substance to be measured including: storing a database with which a relative calibration curve is registered; measuring an internal standard substance having a known concentration; calculating the peak area or the height of an internal standard substance by preparing a chromatogram on the basis of a mass spectrum of the internal standard substance; gaining a mass spectrum of the substance to be measured by measuring the substance; calculating the peak area or the height of the substance by preparing a chromatogram on the basis of the mass spectrum of the substance to be measured; and of calculating the concentration of the substance on the basis of the relative calibration curve, the peak area or the height of the internal standard substance and the peak area or the height of the substance.