To decrease the quantity of cleaning water to be used for water granulated slag and to reduce the cost of a high degree treatment for cooling water including the cleaning water.

Molten slag (a) discharged from a melting furnace is cooled with water to obtain the water granulated slag (b), and after a high temperature waste gas produced in the melting furnace is cooled, dust therein is collected. A part of the cooling water used for the water granulated slag is filtered with a reverse osmosis membrane 1 and the filtrate (d) is used as the cleaning water for the water granulated slag (b). The reverse osmosis membrane removes not only a metallic chloride but also ions, and the filtrate (d) is made harmless as same as industrial water. Water circulation system is formed by using the filtrate (d) as the cleaning water to reduce the consumption of water. The concentrated water (e) from the reverse osmosis membrane 1 is used for the cooling water of the high temperature waste gas. In this way, water in the concentrated water (e) is vaporized to form the dissolved metallic chloride into a fine solid material, which is collected together with other dust, or the like, by a bag filter, or the like.