A linear trap having high ejection efficiency and low ejection energy is realized. In a mass spectrometer in which ions generated by an ion source are introduced to a quadrupole rod structure applied with RF voltage and ejected from the quadrupole rod structure so as to be detected by a detection mechanism, a mass dependent potential is formed in the axial direction of the quadrupole rod structure and ions are ejected mass selectively from the vicinity of a minimum point of the potential, the mass dependent potential being formed by applying electrostatic voltage and RF voltage to an insertion electrode inserted in the quadrupole rods.