To provide a back-filling method which enables transfer even by a vacuum having no agitator and which achieves high uniaxial compressive strength.

Fluidization-treated soil is produced by adding a solidification material to sludge. In this production, a conditioned sludge production process and a mixing treatment process are applied to the sludge. In the conditioned sludge production process, moisture adjustment for the sludge is performed, and the conditioned sludge conditioned to have a specific gravity set to be in the range of 1.18 or higher and less than 1.28 is obtained. In the mixing treatment process, mixing is performed by adding the solidification material in the range of 150 kg or more and less than 260 kg to 1 m³ of the conditioned sludge. The fluidization-treated soil having a uniaxial compressive strength set to be 200 kN/m² or more at a material age of 28 days is obtained by these processes. A fluidization-treated soil m is conveyed, for example, by a vacuum car 1, and filled into a back-filling space by a pump. Thereby, a back-filled state, in which the uniaxial compressive strength at the material age of 28 days is set to be 200 kN/m² or more, is obtained.