Abstract: An ion source is disclosed in which a crucible for holding an ion source material is provided with an aperture in its bottom wall, an emitter chip is disposed within the crucible in a coaxial manner so that the edge of the emitter chip passes through the aperture, a semi-closed crucible made of a conductive material and having the form of a circular cone is disposed in the vicinity of the tip of the emitter chip so as to be coaxial with the emitter chip and to have the same electric potential as the emitter chip, a filament for emitting an electron beam is disposed in the vicinity of the emitter chip, an ion extracting electrode is disposed at a place which is a little spaced apart from the tip of the emitter chip, and a lid is inserted into the ion source material holding part so as to be placed on the above-mentioned semi-closed crucible, thereby preventing the ion source material from being scattered by evaporation.