PURPOSE: To reduce the consumption of heating energy and to eliminate the necessity of partially increasing the flow rate of a refrigerant by furnishing a device for providing a magnetic field in the horizontal direction.

CONSTITUTION: An electron beam 101 is irradiated on the surface of a conductor material 105 in vacuum to melt the conductor material 105, a magnetic field generating coil 106 is provided around the main crucible body 104 for generating vapor, and a DC current is applied. Consequently, a magnetic field is generated, the magnetic field interferes with the natural convection at the molten part of the conductor material 105, Lorentz's force is generated in the direction to control the natural convection, hence the natural convection is controlled, and the heat energy necessary for obtaining the same amt. of evaporation as before can be reduced. Besides, since the heat flux on the cooling surface can be uniformized, the flow rate of a refrigerant need not be partially increased.