To reduce a quantity of heat of exhaust gas, to improve a preheat effect of metallic scrap, and to reduce an amount of cooling water consumption by a method wherein cooling water is circulated through a preheat chamber and refractory materials are laid on the indoor surface side of a preheat chamber wall.

A partition plate 33 to vertically partition the internal gap, and baffle plates 34 to form a serpentine water passage 43 are arranged in an internal gap 32 and further, a feed water port 35 and a drain port 36 communicated with the internal gap 32 are respectively formed. Two ends 41 of a water-cooled pipe 37 are respectively communicated with the internal gap 32 and embedded in unshaped refractory materials 38 laid on the indoor surface side of a preheat chamber. This constitution prevents once-through exhaust gas flowing through the preheat chamber from making direct contact with the water-cooled pipe 37 and an internal wall plate 31, causes the refractory materials 38 to form a heat insulation material, reduces a quantity of heat of which exhaust gas is deprived by cooling water, and also reduces a given amount of passing-through cooling water.