PURPOSE: To prevent any contamination of the surface of an insulating glass by installing a shield on the ion path side of the glass.

CONSTITUTION: A lead-out electrode 3 for leading out an ion beam 2 from an ion source 1 is surrounded by a mirror tube 6 and maintained at plus several kilovolts relative to the earth. Although part of the ion beam 2 led out from the ion source 1 passes through the central hole 7 of the electrode 3, most of the beam 2 is reflected by the surface of the electrode 3. Due to a shield 4, the ion beam 2 reflected by the surface of the electrode 3 does not reach a glass 5. Consequently, there is no possibility that the surface of the glass 5 is contaminated with the reflected ion beam 2 even after long-time use.