To attain the effective stabilization of a waste containing a harmful material such as heavy metal and the excellent physical strength of a treated solidified material.

A treating agent consisting essentially of 20-95wt.% cement mainly containing gypsum such as anhydrous gypsum (CaSO4), calcined gypsum (CaSO4.1/2H2O), gypsum (CaSO4.2H2O) and a powdery porous inorganic absorbent such as a porous silicon dioxide or a porous aluminum silicate is mixed by 3-50 pts.wt. with 100 pts.wt. waste such as an incineration smoke dust containing the harmful material such as lead, cadmium, mercury, chromium, zinc or arsenic, a municipal waste incineration ash such as a main ash or a molten fly ash, an industrial waste incinerate ash, a sludge, a slag or contaminated solid. In such a case water is added if need and kneaded.