To extend a continuous operation time of a cupola as much as possible in the water cooling zone of a cupola and the structure of a furnace bed.

The inner bottom surface (a lower end edge W of the water- cooled zone of a cupola) of a cooling water tub 46 is positioned between a lower limit corresponding to a metal line LM during rest and an upper limit near a slug line LS during normal working. As protection of the cupola is effected, improvement of cooling performance near a slug line most important to corrosion resistance of a furnace side wall 10 is realized. The stamp material layer of a cupola hearth 22 consists of first layers 24a and 24b on the upper surface side formed of a material high in corrosion resistance, and second layers 26a and 26b on the under surface side formed of a material high in heat insulation property. The occurrence and progress of crack of the first layers 24a and 24b are suppressed by the second heat insulation layers 26a and 26b, and life extension can be realized.