PURPOSE: To enable to detect deflection on ion optical path in a mass analyzing system, by applying AC voltage of the same phase overlapping to the DC voltage to the electrodes opposite to each of electrodes of a tetrode lens.

CONSTITUTION: As well as connecting Y direction DC power supply 7Y and X direction DC power supply 7X between the opposite electrodes of a tetrode lens 6 in a mass analyzing system, an AC power supply 8 of the same phase is applied through a switch 9. By overlapping the AC voltage over the DC voltage by turning on the switch 9, the ion displays a deflected peak wave pattern 9 with an effect of the AC voltage, on a display 5 through a detector 4, affected from the AC voltage, while no ion is on the center path of the tetrode lens, by which a deflection can be detected. Therefore, the center path of ion can be readily controlled depending on the deflection value.