To prevent abnormal discharge of an ion source generated in extracting ions from a plasma by setting an electric filed uniformzation shape surrounding a slit in a slit side electrode to a concentric circle about the slit and setting it into a conical shape to relax unwanted electric field on the counter electrode side.

An electric field strength must be intensified for extracting a large number of ions from a plasma and this is incompatible with the reduction of abnormal discharge. However, the generation of abnormal discharge can be suppressed by relaxing the electric field at portions having only a small effect on the extraction of the ions between the opposing electrodes for forming the electric field. A slit-type opening is provided in a part of the space for generating the plasma and a part of the electric field between the electrode on the slit side and the opposite electrode is eased by the ion source having the opposing electrode for generating the electrode field in this opening part. The electrode is set into such a shape as lowering the electric field strength having little effect on ion extraction, even if the ion extracting electrode is got dirty, so that the electric field in an inclined part of a projection of the electrode 2 can be relaxed and the abnormal discharge can be prevented.