PURPOSE: To provide a method for treating blast furnace slags, comprising effecting retention of heat on the upper surface of blast furnace slags while retaining the molten state, compulsorily cooling from the lower surface thereof simultaneously to advance coagulation from the lower surface thereof, whereby gas generated in accordance with the cooling of molten slags is extracted from the upper surface thereof in a molten state to obtain blast-furnace slag ballast having a small porosity and a large specific gravity.