MASS SPECTROMETER

In a tandem mass spectrometer, when the measurement mode is switched between a positive ion measurement mode and a negative ion measurement mode, a DC offset voltage applied to a lens electrode to impart collision energy to an ion is temporarily switched to 0 V (S1). After being maintained at 0 V for a predetermined waiting time (S2), the voltage is changed to a DC offset voltage corresponding to a measurement mode which is used after the switching operation (S3). By such an operation, the voltage difference between the neighboring plate electrodes among the plate electrodes (171, 172, 173) included in the lens electrode can be decreased as compared to the case where the polarity of the DC offset voltage is immediately switched. Consequently, unintended electric discharge between the neighboring electrodes can be prevented.