An ion selection method for selecting an ion having a specific mass-to-charge ratio or an ion group having a specific range of mass-to-charge ratios from among ions captured in an ion trap composed of three or more electrodes (21, 22, 24), the method comprising: a) a rough isolation step, in which ions having a wide range of mass-to-charge ratios including the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are selectively left within the ion trap while other unnecessary ions are removed; b) a low-mass side ion isolation step, in which unnecessary ions, remaining after ion selection by the rough isolation step, having mass-to-charge ratios lower than the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are removed with a separating power higher than in the rough isolation step by performing, on the ions captured in the ion trap, an ion-discharging operation in which a portion of the ions are discharged by shifting a position of a working line on a stability diagram based on a Mathieu equation and thereby changing a lower limit mass that can be captured; and c) a high-mass side ion isolation step, in which unnecessary ions, remaining after ion selection by the rough isolation step, having mass-to-charge ratios higher than the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are removed with a separating power higher than in the rough isolation step by performing, on the ions captured in the ion trap, an ion-discharging operation in which a portion of the ions are discharged without performing a frequency scan by using a resonant excitation using an excitation voltage having a predetermined single frequency, where step a) is performed in advance of steps b) and c), where the two steps b) and c) are performed in this order, in a reverse order or simultaneously. a) a rough isolation step, in which ions having a wide range of mass-to-charge ratios including the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are selectively left within the ion trap while other unnecessary ions are removed; b) a low-mass side ion isolation step, in which unnecessary ions, remaining after ion selection by the rough isolation step, having mass-to-charge ratios lower than the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are removed with a separating power higher than in the rough isolation step by performing, on the ions captured in the ion trap, an ion-discharging operation in which a portion of the ions are discharged by shifting a position of a working line on a stability diagram based on a Mathieu equation and thereby changing a lower limit mass that can be captured; and c) a high-mass side ion isolation step, in which unnecessary ions, remaining after ion selection by the rough isolation step, having mass-to-charge ratios higher than the specific mass-to-charge ratio or the specific range of mass-to-charge ratios to be selected are removed with a separating power higher than in the rough isolation step by performing, on the ions captured in the ion trap, an ion-discharging operation in which a portion of the ions are discharged without performing a frequency scan by using a resonant excitation using an excitation voltage having a predetermined single frequency, where step a) is performed in advance of steps b) and c),