By preventing the trapping efficiency of ions from largely depending on the mass-to-charge ratio, an ion trap mass spectrometer suitable for obtaining a high sensitive mass spectrum is provided. Ions of a specimen to be mass analyzed generated at an external ion source pass through an ion transportation portion and then injected into a space (ion trap volume) between the ring electrode and the end cap electrodes. An RF trap voltage power source applies an RF frequency V.multidot.cos .OMEGA.t between the ring electrode and the end cap electrodes to form a radio frequency electric field in the ion trap volume. The RF trap voltage is changed so that the optimum trap frequency is in inverse proportion to 1/2 power of a mass-to-charge ratio while the ions are being trapped in the radio frequency electric field formed in the ion trap volume.