Abstract: Oxidized proteins, which have undergone oxidative modifications, are labeled with labeling reagents and quantified by mass spectrometry. In this process, a first labeling reagent, which is capable of reacting with the oxidized proteins, and a second labeling reagent, which has the same chemical structure as the first labeling reagent and in which at least part of the component atoms is substituted by an isotope of the atom concerned, are employed as the labeling reagents. The oxidized proteins labeled with the first labeling reagent and the oxidized proteins labeled with the second labeling reagent are mixed together and subjected to mass spectrometry, with mixing ratios varied. As the labeling reagents, are raised here 2,4-dinitrophenylhydrazine and 2,4-dinitrophenylhydrazine in which a carbon atom on the phenyl group has been substituted by a stable isotope (13C), for example.