A plasma ion source includes a discharge chamber in which a plasma is produced by plasma generator, with an acceleration electrode being disposed adjacent to the discharge chamber in order to extract ions from the produced plasma. A deceleration electrode is disposed adjacent to the acceleration electrode to decelerate the extracted ions, and a ground electrode is disposed adjacent to the deceleration electrode. An insulator container is disposed so as to surround the discharge chamber and the respective electrodes, and a shield ring electrode of ground potential is disposed in the vicinity of the deceleration electrode and along an inner wall surface of the insulator container in order to prevent any discharge from arising across the deceleration electrode and the ground electrode.
