PURPOSE: To stabilize the quantity of evaporation, an evaporation direction and vapor density by controlling the height of at least either one of a raw material container and an electron gun to control the incident angle of electron beam to the upper surface of a metal raw material to a definite value.

CONSTITUTION: An electron gun 3 upwardly emitting electron beam 5 for generating metal vapor is arranged on the lateral side of a raw material container 2 upwardly opened having a metal raw material 1 received therein. The electron beam 5 emitted from the electron gun 3 is downwardly deflected by the deflection magnetic field of a magnet 4 for controlling a beam track to irradiate the upper surface of the metal raw material 1. Further, the height of at least either one of the raw material container 2 and the electron gun 3 is controlled by a control means 6 to control the incident angle of the electron beam 5 to the upper surface of the metal raw material 1 to a definite value. By this method, the quantity of evaporation, an evaporation direction and vapor density are stabilized and isotope recovery efficiency can be enhanced.