A switch circuit (4) is turned ON and switch circuits (3, 17, and 18) are turned OFF to apply a negative direct-current voltage from a negative voltage generating unit (2) to a flight tube (5), and while a measurement is conducted, a switch circuit (15) is turned ON, and a capacitor (13) is charged by an auxiliary positive voltage generating unit (11). When the polarity of the applied voltage is switched from negative to positive, the switch circuit (4) is turned OFF and the switch circuit (17) is turned ON to supply a large current from the capacitor (13) to the flight tube (5), thus the capacitance of the flight tube (5) is charged to a positive potential quickly. After that, the switch circuit (17) is turned OFF and the switch circuit (3) is turned ON to apply a stable positive direct-current voltage from a positive voltage generating unit (1) to the flight tube (5). By quickly charging and discharging the capacitance of the flight tube (5) when the positive and negative polarities are switched in this way, a rising time to apply a stable voltage can be shortened even if a restraint is imposed on the maximum output current of the voltage generating units (1 and 2).
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