In a mass spectrometer introducing ions produced at an ion source, and including quadrupole rods which have an inlet and an outlet and to which a radio-frequency voltage is applied, the mass spectrometer, i.e., a mass spectrometry device implemented by a linear trap which exhibits high ejection efficiency, high mass resolution, and low ejection energy, executes the following steps: 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.