PURPOSE: To prevent an accelerating electrode and so forth from being melted, deformed and so forth by detecting a current flowing into the accelerating electrode and an earth current respectively, and then comparing the magnitude of two detected currents for judging the presence or the absence of voltage being impressed on a decelerating electrode.
CONSTITUTION: The presence or the absence of voltage being impressed on a decelerating electrode 6 is judged by detecting an ion takeout accelerating electrode current IE flowing through an ion source and an earth electrode current IT respectively, and then making use of the difference between the values of the IE and IT respectively given when voltage is normally being impressed on a decelerating electrode 6 and when not being impressed on. Besides, the presence or the absence of voltage being impressed on the decelerating electrode 6 is judged by connectively adding a return line independent of a voltage impression line to the decelerating electrode 6, also connecting an adequate resistor in series to the return line and making use of the flowing of a current with its value given through the division of resistance into voltage when the voltage is being normally impressed on the decelerating electrode 6.
KOIKE HIDEKI
ISOGAI SHIZUSHI
