PURPOSE: To prevent an ion-injected material such as a wafer, from being charged up caused by scondary electrons by installing a conductive lattice to cover a target plate on the surface of the target opposite to a filament of the target plate.

CONSTITUTION: A wire net 13 is fixed in a target plate 15 by an insulator 12. The wire net 13 is made wide enough to cover the target plate 15. Since having high energy, primary electrons 2 radiated from a filament 4 pass the meshes of the wire net and collide against the target plate 15. As a result, secondary electrons 3 are radiated from the target plate 15, but since the secondary electrons 3 have lower energy than the primary electrons 2, the secondary electrons 3 are easy to be affected by negative potential of the wire net 13 and their energy decreases. Consequently, the possibility to capture the secondary electrons 3 by ion beam 1 is heightened, so that negative charging-up of a wafer 11 on a disk 10 owing to the secondary electrons 3 themselves which are not captured by ion beam 1 is avoided.