Provided is a method for quantifying vitamin D, with the vitamin D contained in a biological sample being derivatized with a derivatization reagent and being measured with a mass spectrometer, the method including, a derivatization step of derivatizing n number of samples by using n types of 4-(4′-dimethylaminophenyl)-1,2,4-triazoline-3,5-dione (DAPTAD) isotopologues respectively as derivatization reagents, a mixing step of mixing the n types of derivatization samples obtained in the derivatization step, and a quantitative analysis step of subjecting each of the n types of vitamin D derivatives contained in the mixed sample obtained in the mixing step to quantitative analysis using a mass spectrometer.