To improve solid-liquid separation performance, and to obtain high-quality treatment water by forming a flocculated product which is high in density and high in sedimentation, in the flocculation treatment of high-density silt-containing water.

An inorganic flocculant and a cation-based polymer flocculant are added to the silt-containing water, then, an anion-based polymer flocculant is added to the water and the flocculation treatment is applied to the water, and after that, solid-liquid separation is performed. When adding the anion-based polymer flocculant after the cation-based polymer flocculant is added in advance, an SS is rigidity coupled by the action of the inorganic flocculant and the cation-based polymer flocculant, and the high-density and sufficiently-rigid flocculated product is formed while a flocculation diameter is small. After that, by adding the anion-based polymer flocculant, the flocculated product is further coarsened while keeping a high-density state, and thus the flocculated product hardly collapsed and excellent in sedimentation is formed.