PURPOSE: To obtain an excellent heat insulating effect by subjecting the surface of molten metal to rapid diffusion covering and to assure an adequate working environment and to reduce a production cost by suppressing generation of smoke and dust at the time of charging a heat insulating material to the surface of the molten metal.

CONSTITUTION: Lumped chaff of a grain size 0.1 to 30mm formed by subjecting the consolidated matter obtd. by subjecting dry chaff to a pressure mortar treatment and compression heating and molding treatment further to a crushing treatment is used as the heat insulating material. The surface of the molten metal, such as molten steel or molten iron, is covered by the heat insulating material consisting of such lumped chaff. The pressure mortar treatment is executed under conditions of 150 to 600°C and pressure of 1 to 100ton/cm2 at need or is otherwise executed by compressing the dry chaff to a volumetric ratio of 1/8 to 1/15 to form a compact body of a thickness 5 to 30mm and introducing this compact body between two kinds of walls moving relatively apart a spacing 5 to 30mm thereby pulverizing this body.