A mass spectrometric apparatus capable of generating and detecting positive and negative ions stably at the same time. The apparatus preferably comprises: an opening through which a sample gas is introduced; an ion source to generate ions of the sample gas; and a mass spectrometer to analyze the mass of the generated ions. The ion source utilized with the mass spectrometric device comprises: a first needle electrode on which a voltage is applied in order to generate positive ions of the sample gas introduced through the opening; a first counter electrode having a first opening through which the sample gas and the positive ions pass; a second counter electrode disposed opposite the first counter electrode having a second opening through which the sample gas and the positive ions pass; a second needle electrode on which voltage is applied in order to generate negative ions of the sample gas; and a vent through which the sample gas is ejected. Generated ions are then introduced into a vacuum region via an aperture and subjected to mass analysis.