PURPOSE: To improve the durability of a hearth, reduce the damage of the same and permit very easy repairing work by a method wherein the hearth is constituted of a hearth electrode, fixed from the lower side of the hearth, and the layer of MgO-C series mixture, coating the upper side of the electrode.

CONSTITUTION: A plurality of electrodes 5A are penetrated through refractory constituting a hearth 21 while the lower parts of the electrodes 5A are projected out of the hearth 21 to cool them by air. The recess 21A of the hearth 21 is formed on the upper side of the electrodes 5A while the recess 21A is filled with stamp material or MgO-C series mixture powdered body 51A. The MgO-C series mixture, mixed with more than one or two kinds of third constituent such as Cab, SiO2, Fe2O3, Al2O3 or the like, is employed and about 5-25 weight % of C is preferably mixed in the mixture. The thickness of the filling layer of said MgO-C series mixture powdered body 51A is about 20-70% of the whole thickness of the hearth 21. The layer of the MgO-C series mixture 51A has electric conductivity and, therefore, the layer functions as the extended parts of the electrodes 5A and protects the refractory of the hearth 21 in the surrounding of the electrodes.