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