In this invention, the shape and disposition of four rod electrodes (3a to 3d) in a linear type ion trap (3) are offset from an ideal state that forms only a quadrupole electric field. The offsetting is performed in such a manner that: the ratio of an octapole electric field intensity to a quadrupole electric field intensity and the ratio of a dodecapole electric field intensity to the quadrupole electric field intensity have different polarities from one another, the absolute values of each being 0.005 or greater; and the ratio of the octapole electric field intensity to the dodecapole electric field intensity has an absolute value between 0.6 and 1.4. When the quadrupole electric field is superimposed by the octapole electric field and further superimposed by the dodecapole electric field having an opposite polarity from that of the octapole electric field, peak shifts in resonance curves cancel out, yielding a peak having sharp edges at both high frequency and low frequency. Having satisfied these conditions, this linear type ion trap can achieve a high ion trapping efficiency and high ion separation capability.