A plasma source mass spectrometer in which ions in a plasma generated in a high pressure (.ltoreq.1 atm) region are introduced into a low pressure (.ltoreq.10.sup.-5 Torr) region to analysis the ion mass includes a moderate pressure (.gtoreq.10.sup.-3 Torr) region which is provided between the high pressure region and the low pressure region. The plasma generated in the high pressure region is diffused to the moderate pressure region in order to produce a diffused plasma. Ions are extracted from the diffused plasma by an ion extraction electrode having an ion extraction opening. In the vicinity of the ion extraction opening a convex-shaped toward the diffused plasma whereby ion sheath is formed, whereby the ions can be extracted toward the low pressure region with a high efficiency.