The objective of the present invention is to improve the robustness, sensitivity, and maintainability of a mass spectrometer to which a liquid chromatograph can be connected. This mass spectrometer includes: an ion source; a mass spectrometry unit; a plurality of flat plates used as electrodes having openings through which ions can be introduced to the mass spectrometer from an orthogonal direction along an ion introduction axis; a porous member provided between the mass spectrometry unit and the plurality of flat plates; and a mechanism for feeding gas to the plurality of flat plates for the side opposite from the ion introduction direction; the plurality of flat plates being disposed under atmospheric pressure, and the plurality of electrodes being set at a temperature higher than room temperature.