The invention provides a mass spectrometry and a method of the mass spectrometry realizing a linear trap which exhibits high ejection efficiency, high mass resolution, and low ejection energy. The mass spectrometer introduces ions produced at an ion source, and includes quadrupole rods which have an inlet and an outlet and to which a radio-frequency voltage is applied, and the mass spectrometry method includes steps of: trapping at least part of the ions by a trap potential generated on the central axis of a quadrupole field, oscillating part of the trapped ions in an intermediate direction between the mutually-adjacent quadrupole rods, ejecting the oscillated ions by an extraction field, and detecting the ejected ions or introducing the ejected ions into another detection process.