Abstract: An apparatus for manufacturing a rapidly cooled solidified slag, which comprises: at least one rotating cooling drum rotating around a horizontal center axis thereof, said cooling drum being cooled by a cooling medium, and having an outer peripheral surface comprising a plurality of jogs over the entire surface thereof; a slag sump arranged at a position where said peripheral surface of said cooling drum begins to rise, the depositing molten slag onto said peripheral surface of said cooling drum; a scraper proved at the lower part of said peripheral surface of said cooling drum; and a slag feeding means arranged above said cooling drum, for pouring said molten slag into said slag sump; whereby said molten slag poured into said slag sump is deposited onto said peripheral surface of said cooling drum, rapidly cooled through heat exchange with said cooling medium along with rotation of said cooling drum to become a solidified slag, and peeled off from said peripheral surface of said cooling drum by said scraper.