PURPOSE: To reduce power cost and installation cost, by contriving to lower the output of a filament for evaporating metal uranium vapor and reducing energy loss due to heat conduction.

CONSTITUTION: A metal melting crucible 16 is constituted so that the inner surface thereof contacted with metal uranium 5 is formed of a carbon material 7 having m.p. of 3,700±100°C and heat conductivity lower than that of metal uranium, the outside of said carbon material 17 is formed of a zirconium material 18 of which the heat conductivity is further lower than that of the carbon material 17 and the outside of the zirconium material 18 is formed of a graphite material 19 having m.p. of about 3,600°C. Water passing holes 20 for passing cooling water are provided to the graphite material 19 at a plurality of places. As a result, energy loss due to heat conduction of places. As a result, energy loss due to heat conduction at the heating time of metal uranium can be reduced as compared with a conventional means and, therefore, reduction of the output of heating filament can be contrived.