PURPOSE: To produce the pollution-free heat insulating material for annealing furnaces by spinning cotton-form inorganic material composed primarily of Al2O3, SiO2 with heat-resistant yarns as core material, further weaving the same to a woven fabric by a weaving machine or laminating said fabric on a glass met.

CONSTITUTION: Cotton-form inorganic material 23 of short fibers is made by melting raw material composed primarily of Al2O3, SiO2 and with high-velocity flow of steam or air or other methods. This is spun in the form of winding this on any of silica glass yarns, heat-resisting stainless steel yarns 16, tungsten carbide yarns and staple fiber yarns or their composite as core material 24, and these are twisted to make 25 which is then woven by a weaving machine to make a long-length woven fabric 26. Or this woven fabric 26 and heat-resistant silica glass mats 14 are laminated to make a composite form sheet 21. This yields the sealing material of superior heat resistant and heat insulation characteristic which is suited as heat insulating material for annealing furnaces for iron and steel.