PURPOSE: To expedite heat transfer to a vessel and to efficiently melt a raw material by placing the vessel in which the material solidified at a normal temperature is filled in a box, circulating heated air in the box, and supplying steam to the box to condense a surface of the vessel.

CONSTITUTION: A vessel 2 in which raw material is filled is contained from a door 3 into a box 5, and placed on an upper surface of a shelf 4. Then, steam of a high temperature is supplied to a supply tube 6, a motor 13 is simultaneously driven to rotate a fan 11. The steam is distributed from an input side distribution tube 7 to a heating tube 8 to heat the air near one sidewall of the box 5. On the other hand, the heated air is circulated in the box 5 to heat the material in the vessel 2. Thereafter, when the temperature of an inner surface of the box 5 arrives at a predetermined temperature or higher, predetermined steam is supplied from a steam supply tube 12 to the box 5. Thus, the steam is condensed on the surface of the box 5, and its latent heat is applied to the vessel 2 to expedite heat transfer to the vessel 2.