A method wherein a porous body in which a monoclonal antibody to be measured 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, a resulting peptide fragment is detected using liquid chromatography-mass spectrometry (LC-MS), the porous body has an average pore diameter in a range of 10 nm - 200 nm, the nanoparticles have an average particle size in a range of 50 nm - 500 nm, the average particle size of the nanoparticles is larger than the average pore diameter of the porous body, and a peptide fragment to be detected has 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, wherein the monoclonal antibody is trastuzumab or trastuzumab-DM1, bevacizumab, or rituximab, and a detection target is a peptide having an amino acid sequence expressed in SEQ ID No: 1, 3, 6 and/or 7 when the monoclonal antibody is trastuzumab or trastuzumab-DM1, SEQ ID No: 9 - 11 when the monoclonal antibody is bevacizumab, or SEQ ID No: 13, 15 and/or 16 when the monoclonal antibody is rituximab, and wherein the detection target is detected by multiple reaction monitoring under the following conditions Peptide Sequence Parent ion m/z Fragment ion m/z Retention time [minutes] Q1 Pre Bias Collision Energy Q3 Pre Bias IYPTNGYTR 542.8 808.4 3.405 -40 -18 -30 IYPTNGYTR 542.8 711.3 3.405 -40 -26 -38 IYPTNGYTR 542.8 610.3 3.405 -40 -24 -22 IYPTNGYTR 542.8 404.7 3.405 -20 -18 -15 IYPTNGYTR 542.8 277.2 3.405 -20 -16 -30 FTISADTSK 485.2 822.4 3.635 -18 -18 -32 FTISADTSK 485.2 721.4 3.635 -11 -18 -26 FTISADTSK 485.2 608.3 3.635 -18 -19 -32 FTISADTSK 485.2 521.3 3.635 -18 -19 -38 FTISADTSK 485.2 335.2 3.635 -18 -24 -24 GLEWVAR 415.7 660.3 4.077 -30 -15 -24 GLEWVAR 415.7 531.3 4.077 -30 -16 -40 GLEWVAR 415.7 345.2 4.077 -16 -17 -24 GLEWVAR 415.7 246.2 4.077 -30 -18 -17 NTAYLQMNSLR 655.8 1095.6 4.144 -24 -23 -42 NTAYLQMNSLR 655.8 1024.5 4.144 -24 -22 -40 NTAYLQMNSLR 655.8 861.5 4.144 -24 -22 -32 NTAYLQMNSLR 655.8 748.4 4.144 -24 -23 -28 NTAYLQMNSLR 655.8 620.3 4.144 -24 -23 -32 NTAYLQMNSLR 655.8 489.3 4.144 -24 -23 -25 NTAYLQMNSLR 655.8 375.2 4.144 -24 -22 -19 when the monoclonal antibody is trastuzumab or trastuzumab-DM1, under the following conditions Peptide Sequence Parent ion m/z Fragment ion m/z Retention time [minutes] Q1 Pre Bias Collision Energy Q3 Pre Bias STAYLQMNSLR 642.3 1095.6 4.147 -24 -24 -42 STAYLQMNSLR 642.3 1024.5 4.147 -24 -25 -40 STAYLQMNSLR 642.3 861.5 4.147 -24 -25 -20 STAYLQMNSLR 642.3 748.4 4.147 -24 -22 -28 STAYLQMNSLR 642.3 620.3 4.147 -24 -24 -32 STAYLQMNSLR 642.3 489.3 4.147 -24 -22 -25 STAYLQMNSLR 642.3 375.2 4.147 -24 -22 -27 FTFSLDTSK 523.3 898.5 4.432 -38 -20 -34 FTFSLDTSK 523.3 797.4 4.432 -38 -18 -30 FTFSLDTSK 523.3 650.3 4.432 -38 -19 -34 FTFSLDTSK 523.3 563.3 4.432 -38 -22 -40 FTFSLDTSK 523.3 450.2 4.432 -20 -23 -30 FTFSLDTSK 523.3 335.2 4.432 -20 -25 -24 FTFSLDTSK 523.3 399.2 4.432 -38 -17 -11 FTFSLDTSK 523.3 249.1 4.432 -38 -17 -17 VLIYFTSSLHSGVPSR 588.3 832.4 4.515 -22 -19 -30 VLIYFTSSLHSGVPSR 588.3 775.9 4.515 -22 -18 -28 VLIYFTSSLHSGVPSR 588.3 719.4 4.515 -22 -18 -26 VLIYFTSSLHSGVPSR 588.3 637.8 4.515 -22 -18 -24 VLIYFTSSLHSGVPSR 588.3 564.3 4.515 -22 -22 -20 VLIYFTSSLHSGVPSR 588.3 602.3 4.515 -22 -28 -22 VLIYFTSSLHSGVPSR 588.3 359.2 4.515 -22 -26 -26 VLIYFTSSLHSGVPSR 588.3 213.2 4.515 -22 -20 -23 when the monoclonal antibody is bevacizumab, or under the following conditions Peptide Sequence Parent ion m/z Fragment ion m/z Retention time [minutes] Q1 Pre Bias Collision Energy Q3 Pre Bias ATLTADK 360.2 648.4 2.809 -26 -14 -24 ATLTADK 360.2 547.3 2.81 -13 -13 -20 ATLTADK 360.2 434.2 2.81 -26 -16 -22 ATLTADK 360.2 333.2 2.81 -26 -19 -24 ATLTADK 360.2 262.1 2.81 -13 -21 -29 LASGVPVR 399.7 685.4 3.426 -29 -17 -34 LASGVPVR 399.7 614.4 3.426 -30 -16 -32 LASGVPVR 399.7 527.3 3.426 -15 -16 -38 LASGVPVR 399.7 371.2 3.426 -15 -17 -28 ATSNLASGVPVR 586.3 912.5 3.728 -22 -25 -34 ATSNLASGVPVR 586.3 685.4 3.728 -22 -23 -38 ATSNLASGVPVR 586.3 614.4 3.728 -22 -25 -24 ATSNLASGVPVR 586.3 527.3 3.728 -22 -25 -40 ATSNLASGVPVR 586.3 470.3 3.728 -22 -19 -17 ATSNLASGVPVR 586.3 371.2 3.728 -22 -17 -27 when the monoclonal antibody is rituximab.