PURPOSE: To generate a high intensity ion beam by feeding a liquid material to the tip of a needle-like member, by forming a high electric field so as to produce a sharp conical protrusion, and by cooling the needle-like member to solidify it.

CONSTITUTION: When a voltage is applied across a needle-like member 4 and a leader electrode 5, an electric field is concentrated at the needle tip, and gallium 3 in a reservoir 1 is extracted to the tip through the fine hole 2 at the bottom by the strong electric field, thus forming a conical protrusion called a Taylor's cone. Under this condition, the inside of the reservoir 1 and further the liquid gallium 3 are gradually cooled through a stanchion 9, support member 8, reservoir 1, etc. by the feeding of the refrigerant, and the gallium 3 is solidified at its melting point (about 30°C) or lower while maintaining an approximately conical shape, thereby forming a strong electric field near its tip. Then, a valve 13 is opened so as to introduce argon gas into an ionization chamber from a gas cylinder, and the gas is ionized by the strong electric field and an argon ion beam is available.