To obtain slag solidified in such a shape as to yield slag aggregate having a proper shape and a proper grain size distribution after crushing at a low cost, without increasing the solidification thickness of slag beyond necessity.

Different rugged shapes are separately provided to the upper and lower surfaces of solidified slag formed by pouring slag from a blast furnace into a continuously conveyed casting mold. Whereby, since a targeted grain size distribution of slag aggregate can be directly made while suppressing thermal load applied to the casting mold, slag solidified in such a shape as to yield slag aggregate having a proper shape and a proper grain size distribution after crushing at a low cost can be obtained, without increasing the solidification thickness of slag beyond necessity.