To improve the durability of a melting furnace and achieve a continuous safe operation by forming a furnace wall part sequentially with fire resistant bricks having a prescribed bulk specific gravity and an apparent porosity, a heat resistant and heat insulating material having a prescribed bulk specific gravity in contact with the heat resistant bricks and a steel plate in contact with the heat resistant and heat insulating material from the inside of the furnace to the outside of the furnace.

A furnace wall part in the lower part of the side wall of a furnace main body 11 in contact with a molten material 21 is formed sequentially with fire resistant bricks 31 having a bulk specific gravity not lower than 2.7 and an apparent porosity not higher than 12%, a heat resistant and insulating material 32 having a bulk specific gravity not lower than 1.0 in contact with the fire resistant bricks 31 and a steel plate 33 in contact with the heat resistant and insulating material 32 from the inside of the furnace to the outside of the furnace. Further, the furnace wall part in the upper part of the side wall of the furnace main body 11 in contact with gas 26 is formed sequentially with an undefined heat resistant castable material 41 having a bulk specific gravity not lower than 2.7, a fire resistant and heat insulating material 42 having a bulk specific gravity not lower than 1.0 in contact with the heat resistant castable material 41 and a steel plate in contact with the material 42 from the inside to the outside of the furnace. Thus, a molten salt is prevented from penetrating the heat resistant and insulating material through heat resistant bricks.