PURPOSE: To facilitate the heating of a needle as well as to make a liquid metal ionizable even if it has a high melting point, by constituting a reservoir itself storing liquid metal as a V-shaped tubular filament, while installing a fine hole in this V-shaped bent part and inserting the needle in this hole, then welding and fixing it while exposing the one end out.

CONSTITUTION: A tubular filament 2 consisting of W, Ta or the like is bent in a V-shaped form at the central part, in which a fine hole 9 for needle insertion is opened, and then a liquid metal 3 which can produce an ion beam is filled up inside the filament 2. Next, a needle 1 consisting of W or the like is inserted into the fine hole 9, having the lower end exposed out to some extent, and the other end 1a is bent, welded to the filament 2, and electrodes 12 and 13 supported by a glass insulating plate 15 are inserted into both openings of the filament respectively, while a filament heating power source 14 is connected to these electrodes. In addition, an electrode 5 for drawing out the ion beam is set up as being opposed to the needle 1, and a high-tension power source 16 is converted between the electrode 5 and the electrode 13. Thus, the needle is directly heated by the liquid metal, whereby ionization is facilitated.