In a three-dimensional quadrupole-type ion trap, a shape and an arrangement of the ring electrode and the end cap electrodes 11 and 12 are shifted from an ideal state in which only a quadrupole electric field is formed, so that the polarities of the ratio of strength of an octupole electric field with respect to the strength of a quadrupole electric field and the ratio of strength of a dodecapole electric field with respect to the strength of the quadrupole electric field are different from each other, their absolute values are equal to or greater than 0.02, and the absolute value of the ratio of strength of the octupole electric field with respect to the strength of the dodecapole electric field is within the range of from 0.6 to 1.4.