A mass analysis is initially performed while applying appropriate voltages to the electrodes so that ions injected through an entrance gate electrode (5) into a loop orbit (3) are guided through approximately one half of the loop orbit (3) and diverted at an exit gate electrode (6) toward an ion detector (7). Based on the intensities of the peaks appearing on a mass spectrum obtained by this mass analysis, one or more objective ions are selected and a time parameter is specified so that the voltage applied to the exit gate electrode (6) changes when none of the ions flying along the loop orbit (3) are passing through the exit gate electrode (6). As a result, the orbit of the objective ions will assuredly changed so that they will be directed toward the ion detector (7) after flying through the loop orbit (3) multiple times. Thus, the mass information of the objective ions can be assuredly obtained.