To provide a method for producing and back-filling a liquefied stabilized soil, which can be carried not only by an agitator truck with a stirring device but also by a vacuum truck without having the stirring device, and can attain a high uniaxial compression strength.

In the method for producing the liquefied stabilized soil by adding a solidifying material such as cement to sludge, the sludge is subjected to an adjusted-sludge producing process and a kneading processing process. In the adjusted-sludge producing process, the sludge is subjected to moisture control to obtain an adjusted-sludge controlled in moisture content to 1.18 or more and less than 1.28 in specific gravity. Further, in the kneading processing process, the solidifying material is added within a range of 150 kg or more and less than 260 kg per 1 m3 of the adjusted-sludge. These processes produce the liquefied stabilized soil of 200 kN/m2 or more in the uniaxial compression strength at a material age of 28 days. The liquefied stabilized soil m thus produced can be carried on the vacuum truck 1.