PURPOSE: To lower heat loss and heighten evaporation efficiency by irradiating a porous tungsten with an electron beam which contains bubbles and in which a metal for evaporation is impregnated.

CONSTITUTION: For example, porous tungsten mixture 2 which contains bubbles and in which a metal such as uranium is impregnated is put in a crucible 3 and the surface of the mixture is irradiated with an electron beam 4. Temperature of the electron beam irradiated spot increases and thereby uranium impregnated in the mixture 2 is evaporated. As a result, equivalent thermal transmittance is lowered owing to the bubble and heat loss to the crucible cooled by water is lowered and thus evaporation efficiency is improved.