PURPOSE: To keep the space charge effect of an ion beam nearly constant and suppress the fluctuation of the beam focus point position when operating parameters are changed by providing electron feed electrodes to feed secondary electrons into an ion beam, and moving them back and forth with respect to the center of the ion beam.

CONSTITUTION: Electron feed electrodes 12 hit by the periphery section of an ion beam 4 to emit secondary electrons and feeding them into the ion beam 4 is provided on a beam transportation line between an ion source 2 and an analyzing electromagnet 6. They are two parallel flat plates arranged along the advance direction of the ion beam 4, and both plates are moved by a driving device 14 in package in the longitudinal direction with respect to the center of the ion beam 4. The effective space charge effect added with both actions of positive ions and electrons in this segment can be kept nearly constant when operating parameters affecting the space charge effect such as the ion type, energy, and beam quantity of the ion beam 4 and the degree of vacuum of the beam transportation line are changed.