An electrostatic Kingdon ion trap in which ions can oscillate harmonically in the longitudinal direction, decoupled from their motions in the transverse direction is formed from at least three inner electrodes located inside a hollow outer housing electrode. The inner surface of the housing electrode and the outer surfaces of the inner electrodes are formed so that when a potential is applied between the housing and the inner electrodes, the potential distribution inside the housing contains not only a term for a harmonic potential well in the axial direction, but also a term for the potential distribution in the radial direction, that contains, independent of the axial coordinate, the equations for a family of Cassini curves of at least the third order.