To prevent the development of molten powdery dust as much as possible and to efficiently recovery metal in slag.

This slag treating apparatus is constituted of a steel plate hearth 12 incliningly arranged at 3-7° angle and having ≥10 mm thickness, a shower nozzle 13 for spouting cooling water as spraying state onto the steel plate hearth 12, a water ejecting gun 14 arranged at the upstream side of the steel plate hearth 12 and jetting jet water on the slag formed as powder state on the steel plate hearth 12 with the spreading state water and push-flowing to the downstream side, a screen 16 arranged at the downstream side of the steel plate hearth 12 and screened into the powdery slag push-flowing with the water ejecting gun 14 and the metal, a collecting pit 17 arranged at the lower part of the screen 16 and a water collecting pit for collecting the cooling water. The molten slag 2 flows as thin state on the steel plate hearth 12 and the cooling water is spouted as the spraying state onto the flowing slag and the slag is formed as the powder state on the steel plate hearth 12. Thereafter, the jet water is ejected to the powdery slag and the slag having minus mesh through the screen 16, is collected into the collecting pit 17. The cooling water is collected into the water collecting pit.