PURPOSE: To secure a better focusing property of an ion beam by constituting an ion extracting system of a screen grid, a plurality of accelerating grids and a plurality of decelerating grids, the ion extracting system extracting generated ions by means of acceleration.

CONSTITUTION: A porous screen grid 2 for forming an ion emitting surface, a porous accelerating grid 3 for extracting and taking out only ions as a focusing beam from the ion emitting surface by means of acceleration, a porous decelerating grid 4 for providing space potential to ions emitted are disposed on one face of an ion generating chamber 1 and an ion extracting system comprises these grids. In this case, a second accelerating grid 10 is added to the accelerating grid 3. A stepwise accelerating voltage is applied to each of the accelerating grids 3, 10 and the focusing property of ions is enhanced by the effect of an electron lens. Reduction of grid abrasion caused by non-focused ions, etc., is thereby achieved.