While applying a square wave voltage to the ion electrode (21) so that ions already captured in the ion trap (20) do not disperse, the timing of irradiating a laser light for ion generation is controlled in such a manner that ions reach the ion inlet (25) at a predetermined timing of a cycle of the voltage. In the case of a positive ion (cation) for example, the timing of laser light irradiation is adjusted in such a manner that the target ions reach the ion inlet (25) in the low level period of a cycle of the square wave voltage. By injecting ions in addition to the ions already captured in the ion trap (20) in this manner, the amount of ions can be increased, and by performing a mass separation and detection after that, the signal intensity in one mass analysis can be increased. Accordingly, by decreasing the number of repetitions of the mass analysis for summing up mass profiles, the measuring time can be shortened.