A method is provided for more easily detecting and quantifying a protein by regioselectively digesting a variable region of an Fab domain of a monoclonal antibody while suppressing proteolysis of an Fc domain.In the method, a porous body in which a monoclonal antibody is immobilized in pores and nanoparticles on which a protease is immobilized are brought into contact with each other in a liquid to perform selective proteolysis of the monoclonal antibody, and a resulting peptide fragment is detected using liquid chromatography-mass spectrometry (LC-MS), and a peptide having an amino acid sequence that includes an amino acid derived from a CDR2 region of a heavy chain or a light chain of the monoclonal antibody is detected.