PURPOSE: To insulate the heat of a vessel for containing a molten iron at a low cost by charging a carbon source into said vessel which has the sensible heat during the standing-by till the next use so as to burn said source in the vessel thereby maintaining the high temp. in the vessel.

CONSTITUTION: The carbon source 4 such as powder coke or coal is sprayed to the bottom, inside wall, etc. in a converter 1 and a proper amt. of air is supplied therein from an air supply pipe 2 to burn the carbon source. The inside of the converter is coated approximately uniformly by a combustion flame 3 generated by such combustion so as to have the uniform temp. in the converter. The converter 1 having the sensible heat during the standing-by is thus made to maintain the adequate temp. by which the failure of the refractories in the converter is prevented with the inexpensive fuel and the smooth starting in the stage of the next use is made possible.