A method for measuring the absolute amount of a target protein contained in a protein mixture, including the steps of: (A) fragmenting the sample containing the target protein and labeling with a stable isotope X; (B) adding a known amount of an internal standard that is prepared by fragmenting a standard protein that is identical to the target protein, and labeling with a stable isotope Y; (C) placing the sample in an LC-MS/MS device and performing multiple reaction monitoring (MRM) analysis using MRM transitions selected for the internal standard; and (D) identifying a peptide derived from the target protein (a target peptide) that shows the same retention time as a peptide derived from the internal standard (an internal standard peptide), and quantifying the target protein in the test sample by comparing the peak area of the internal standard peptide with the peak area of the target peptide.