A mass spectrometer using a plasma ion source for analyzing an ultra-trace element includes a plasma generation system for generating a plasma including the composition of a sample, an ion beam formation system for extracting ions in the form of a beam from the plasma generating the ions, a mass spectrometry system for performing mass spectrometry of the ion beamn, and an ion detection system for detecting the ions subjected to the mass spectrometry, in which a lens system made up of a cylindrical first electrode, a cylindrical second electrode with a photon stopper disposed on the central axis thereof, and a cylindrical third electrode is further provided between the ion beam formation system and the mass spectrometry system. By the provision of the lens system, the ions generated in the plasma are transported more efficiently to the side of the mass spectrometry system and by the provision of the photon stopper in the above described position, it is achieved, with a simpler structure, to prevent photons from entering the ion detection system.