PURPOSE: To arrest the high heat of slag effectively and utilize the slag most effectively as a heat generating body capable of being moved and reserved by a method wherein the thick plate of slaked lime is received in a reacting vessel made of copper while molten slag is covered while pouring it on the plate of slaked lime through a regulating tank.

CONSTITUTION: High-temperature molten slag 1, discharged onto a slag carrying truck from a smelting furnace, is poured on the thick plate 4 of slaked lime, which is received in a box type reacting vessel 3 made of copper and is moved on a belt conveyer V at a proper speed, through a copper made discharging tank 2 whose discharging rate can be controlled properly whereby the slag 1 is covered on the plate 4 of slaked lime. In this case, the thickness of the slaked lime plate 4 is regulated to a value capable of reacting the high temperature of the slag 1 sufficiently. The slag 1 is poured while keeping the temperature thereof as high as possible than the starting temperature of solidification of the same whereby remaining moisture in the slaked lime plate 4 in the reacting vessel 3 is released perfectly, the plate 4 is heated by contact with the high-temperature slag 1 and the slaked lime is changed into caustic lime.