In an ion cyclotron resonance mass spectrometer in which ions are trapped axially by applying electrical potentials to a pattern of electrode elements to produce an inhomogeneous alternating radio-frequency electric field with a repulsive effect, an additional electrostatic ion-attracting field is superimposed on the repulsive electric field. The voltage of the ion-attracting field is adjusted to compensate for a cyclotron frequency shift of the ions caused by the ion space charge. The voltage of the ion-attracting field can be adjusted so that the ion cyclotron frequency of all ions becomes independent of the number of ions inside the spectrometer.