A mass spectrometer having: an ion trap composed of a ring electrode and a pair of end cap electrodes (32, 34); and a time-of-flight mass spectrometer unit for mass analyzing ions ejected from the ion trap, the mass spectrometer comprising: a1) an end cap voltage applier for selectively applying a radio-frequency high voltage having an amplitude of 100V or more and a direct-current voltage to the end cap electrodes (32, 34); a2) a ring voltage applier for applying an ion-capturing radio-frequency high voltage having an amplitude of 100V or more to the ring electrode; b) a gas introducer for introducing a cooling gas into the ion trap; and c) a controller arranged and adapted to c1) apply the ion-capturing radio-frequency high voltage to the ring electrode by the ring voltage applier, and then c2) conduct a cooling of ions by introducing a cooling gas into the ion trap by the gas introducer while ions to be analyzed are captured in the ion trap and applying the radio-frequency high voltage to the end cap electrodes (32, 34) by the end cap voltage applier while the application of the ion-capturing radio-frequency high voltage to the ring electrode applied by the ring voltage applier is halted, and then c3) apply the direct-current voltage to the end cap electrodes (32, 34) by the end-cap voltage applier to give a kinetic energy to the ions to eject the ions from the ion trap. an ion trap composed of a ring electrode and a pair of end cap electrodes (32, 34); and a time-of-flight mass spectrometer unit for mass analyzing ions ejected from the ion trap, the mass spectrometer comprising: a1) an end cap voltage applier for selectively applying a radio-frequency high voltage having an amplitude of 100V or more and a direct-current voltage to the end cap electrodes (32, 34); a2) a ring voltage applier for applying an ion-capturing radio-frequency high voltage having an amplitude of 100V or more to the ring electrode; b) a gas introducer for introducing a cooling gas into the ion trap; and c) a controller arranged and adapted to c1) apply the ion-capturing radio-frequency high voltage to the ring electrode by the ring voltage applier, and then c2) conduct a cooling of ions by introducing a cooling gas into the ion trap by the gas introducer while ions to be analyzed are captured in the ion trap and applying the radio-frequency high voltage to the end cap electrodes (32, 34) by the end cap voltage applier while the application of the ion-capturing radio-frequency high voltage to the ring electrode applied by the ring voltage applier is halted, and then c3) apply the direct-current voltage to the end cap electrodes (32, 34) by the end-cap voltage applier to give a kinetic energy to the ions to eject the ions from the ion trap. a1) an end cap voltage applier for selectively applying a radio-frequency high voltage having an amplitude of 100V or more and a direct-current voltage to the end cap electrodes (32, 34); a2) a ring voltage applier for applying an ion-capturing radio-frequency high voltage having an amplitude of 100V or more to the ring electrode; b) a gas introducer for introducing a cooling gas into the ion trap; and c) a controller arranged and adapted to c1) apply the ion-capturing radio-frequency high voltage to the ring electrode by the ring voltage applier, and then c2) conduct a cooling of ions by introducing a cooling gas into the ion trap by the gas introducer while ions to be analyzed are captured in the ion trap and applying the radio-frequency high voltage to the end cap electrodes (32, 34) by the end cap voltage applier while the application of the ion-capturing radio-frequency high voltage to the ring electrode applied by the ring voltage applier is halted, and then c3) apply the direct-current voltage to the end cap electrodes (32, 34) by the end-cap voltage applier to give a kinetic energy to the ions to eject the ions from the ion trap.