PROBLEM TO BE SOLVED: To facilitate modification/solidification by adding earth of a specified water content or sand into mud earth of a high water content, mud water, or sludge for mud conditioning and pretreatment. SOLUTION: In the modification/solidification of mud earth of a high water content, mud water, and sludge generated in work sites of dredging, tunnel construction, and others, earth 50 wt.% or below in water content or sand is added for mud conditioning and pretreatment. The amount of the earth or the sand to be added is set up to be 100-5000 kg per m<3> of the mud earth, mud water, or the sludge. Additionally, a polymer-type modifier, a cement-type modifier, and a lime-type modifier can be added. Moreover, one or more than two kinds of substances selected from the group consisting of a botanical absorbent, a fibrous substance, an inorganic porous substance, and a swellable clay mineral can be added. In this way, the fluidity of the materials is lost in a short time, and treated earth is obtained which is high in strength and easy of recycling or disposal.
