An apparatus for manufacturing a rapidly cooled solidified slag, which comprises: a pair of cooling drums, the peripheral surfaces of said pair of cooling drums being in contact with each other and rotating in directions opposite to each other at the same peripheral speed; a pair of weirs provided at the both ends of said pair of cooling drums so as to be in contact with the both ends of said pair of cooling drums, said pair of weirs forming a slag sump in cooperation with the upper half of the peripheral surface of each of said pair of cooling drums, and a molten slag being poured into said slag sump; a pair of gas nozzles provided adjacent the peripheral surface of each of said pair of cooling drums, said pair of gas nozzles uniformly ejecting a gas onto the molten slag, which has been deposited on the surface of said pair of cooling drums in said slag sump, to blow part of said molten slag back into said slag sump, thereby uniformly reducing the thickness of said molten slag deposited on the peripheral surface of said pair of cooling drums; a cooling medium for cooling said pair of cooling drums, said cooling medium being supplied to each of said pair of cooling drums, and exchanging heat with said molten slag deposited on the peripheral surfaces of said pair of cooling drums, whereby said molten slag becomes a rapidly cooled solidified slag through heat exchange with said cooling medium, and is peeled off from the peripheral surfaces of said pair of cooling drums by means of a scraper.