PURPOSE: To improve the treating speed of molten blast furnace slag without deterioration of the quality by passing said slag between a circular moving bed and special rotating rolls and rolling and solidifying the slag.

CONSTITUTION: Moletn blast furnace slag 9 is supplied onto a circular moving bed 1 of a turntable type, etc. The slag is solidified by the cooling water ejected from a water sprayer 2 and the cooling water supplied into rotating rolls 3 while the slag is rolled between the bed 1 and the rolls 3, by which the slag is made into thin sheet-like rolled slag 10. Such slag is crushed by a coarse crushing roll 6 and is discharged onto a conveyor 8. The bed and the rolls 3 expand by receiving the heat of the molten slag 9 but the transverse central part of the rotating rolls are preliminarily formed into a recessed shape and therefore the rotating rolls compensate the deforming component thereof. The rolled slag having substantially a uniform thickness is consequently obtd. The rolled slag is usable suitably as raw material, for cement etc.