Abstract: An apparatus for manufacturing a vitreous slag, which comprises: a pair of cooling drums, the peripheral surfaces of said pair of cooling drums being in contact with each other, and said pair of cooling drums rotating in directions opposite to each other at the same peripheral speed; a pair of weirs provided at the upper halves of the both ends of said pair of cooling drums so as to be in contact with said both ends of said pair of cooling drums, a slag sump being formed by means of said pair of weirs and the bodies of said pair of cooling drums, and molten slag being poured into said slag sump; a cooling medium for cooling said pair of cooling drums, said cooling medium comprising a high boiling point heat medium having a boiling point under 1 atmospheric pressure of at least 200.degree. C.