The invention is provides a method and an apparatus for performing electrospray ionization mass spectrometric analysis, which method can exactly analyze a mass of a molecular ion or a fragment ion of an unstable organometallic complex or a polymeric organic compound. The method includes cooling a sample to be analyzed which contains a solvent and is caused to flow out from a spray small-diameter tube with an inert gas for low temperature vaporization (A), and carrying out ionization and mass spectrometric analysis of the sample to be analyzed while cooling a chamber (7) for removing a solvent or an ion source shield (8) with liquid nitrogen.