The present invention provides a method in which a porous body having a monoclonal antibody to be measured immobilized in pores thereof is brought into contact with nanoparticles having a protease immobilized thereonto in a liquid to perform selective protease digestion of the monoclonal antibody and a peptide fragment obtained by the digestion is detected by liquid chromatography mass spectrometry (LC-MS), wherein the monoclonal antibody is digested with the protease in the presence of an antibody specifically binding to the monoclonal antibody or a target molecule of the monoclonal antibody.