A method of solidifying and detoxifying a soil, incinerated ash and coal ash to yield a solidified substance detoxified to a level complying with the environmental standards and a method of detoxifying polluted water containing dioxins and polluted water containing PCB are provided. First solidifying and detoxifying agent contains light burnt magnesite as component (1) and chrysolite powder as component (2), in which a ratio by weight of the component (1) and the component (2) is 1:5 to 5:1, preferably 1:3 to 3:1, more preferably 1:2 to 2:1 and is the solidifying and detoxifying agent applied to one kind selected from soil, incinerated ash and coal ash as well as polluted water containing dioxins and polluted water containing PCB. Other solidifying and detoxifying agent further contains as an additional component a fused phosphate fertilizer and titanium dioxide powder as well as anhydrous gypsum powder, light burnt dolomite and/or calcined lime powder.