PURPOSE: To selectively omit ions and electrons and to supply only neutral atoms to an isotope separating part in an isotope separation apparatus by laser by mounting a positive charged and a negative charged electrodes in the vapor outlet part of a metallic vapor generator by the impact of electron beam.

CONSTITUTION: A metal 3 in a crucible 3 is heated and melted by a electron beam 2, and neutral atoms, positive charged ions and negative charged ions are generated and fly about in the crucible 4 and a collimater 8. In this case, ions are returned back in the crucible by the positive charged electrode 6 and electrons are attracted and stuck to the electrode 6. Moreover, the rests of the ions and the electrons are attracted to stick and returned back respectively by the negative charged electrode mounted at an upper part. Thus, the atom beam 9 supplied to the isotope separating part 11 from the collimater 8 is contained only neutral atoms. This vapor generating vessel is especially suitable to the generation of atom beam of uranium metal.