PURPOSE: To throttle the primary ion beam thin by furnishing No.1 lens functioning as a deceleration lens for primary ion beam and as an acceleration lens for the secondary, and providing No.2 lens which functions as an acceleration lens for the primary ion beam and as deceleration lens for the secondary.

CONSTITUTION: The primary ion beam emitted from an ion source is deflected by deflectors D1, D2, follows a track 1, 2, or 3, and passes through No.1 lens L11 and a drawout slit 21 to be incident approx. perpendicularly to the surface 20 of a specimen. Therein the No.1 lens L11 functions as a deceleration lens for primary ion beam and as an acceleration lens for the secondary and is used in combination with the No.2 lens L12 which functions as an acceleration lens for the primary ion beam and as deceleration lens for the secondary. This enables throttling the primary ion beam thin.