PURPOSE: To facilitate mass spectrometry by performing the mass spectrometry after changing an objective element, provided in a filament, into oxidized object in the oxidizing atmosphere, so that the element can be ionized.

CONSTITUTION: A cryopump 2a, turbomolecular pump 2b and a rotary pump 2c are connected to a vacuum chamber, to vacuum-exhaust its inside. Then by a mass flow meter, air is introduced into the vacuum chamber via an air introducing pipe, to lower down a vacuum degree. In a condition of holding this vacuum degree, a filament is electrified from a DC power supply in the oxidizing atmosphere, to oxidize zinc in a filament surface into zinc oxide. After ending electrification, introducing air is stopped to increase the vacuum degree by again using an exhausting pump 2, and when the filament is electrified, a zinc ion can be detected by a prescribed current value. Then by oxidizing just before analysis, ionization can be performed, and mass spectrometry can be facilitated.