It is intended to provide an analysis method for a biological sample with clinical significance, which is a system being simple to such an extent that it can be applied to a clinical field, and capable of capping a molecule ionization suppression in mass spectrometry. Mass spectrometry of a biological sample using immunoprecipitation, comprising the steps of: preparing a sample containing an objective biological molecule; concentrating the biological molecule in the sample by immunoprecipitation using an antibody-bound carrier or a carrier to which a molecule capable of specifically binding to an antibody is bound; washing an immunoprecipitate of the biological molecule by using an aqueous solution containing a charge neutralizing agent as a washing liquid; and detecting the biological molecule by subjecting the washed immunoprecipitate to mass spectrometry.