PURPOSE: To form fibers from a blast furnace slag, etc. without requiring remelting, by mixing coke or molten iron in a molten slag, heating the mixture with a burner using air enriched with oxygen as a combustion air while bubbling the air enriched with oxygen into the molten mixture, and forming fibers from the mixture.

CONSTITUTION: A high-temperature molten slag 9 discharged from a furnace is introduced into a heating vessel 1, and an air blowing lance 6 is inserted in the molten slag 9 at a high temperature. Coke or molten iron is then mixed with the slag 9, and the temperature of the molten slag 9 is increased to about 1,400W 1,500°C by heating burners 4 using air enriched with oxygen as a combustion air while bubbling air enriched with oxygen through the lance 6 into the molten slag 9. A raw material for adjusting the composition, e.g. Al2O3, SiO2, MgO or Na2O, is introduced from an introductory port 3 provided in a lid 2 into the slag 9 to adjust the composition, and slag fibers are then formed by the blowing, centrifugal force method, etc.