To modify stainless steel slag and prevent its powdering while securing safety during work without affecting molten steel quality.

A slag-receiving pot is charged with waste concrete material having bound water so as to be 1-10wt.% based on the total weight of stainless steel slag after completion of its charging, and the molten stainless steel slag <1.87 in basicity is charged on the waste concrete material. Besides, 0.3-3wt.%, based on the total weight of the whole material, of a boric acid compound is used combinedly with the waste concrete material.