To provide a solidifying agent for water-containing sludge stabilizing treatment developing strength enough to subject bottom material sludge, which exceeds 10% in its heating weight loss and contains a large amount of organic matter, to recycling as soil for construction such as back filling construction or the like, and a solidifying method for the stabilizing treatment of water-containing sludge.

An anionic water-soluble polymer and an aluminum salt are added to the water-containing sludge to stirr the water-containing sludge and a hydraulic substance is added to or mixed with the stirred mixture. Further, the solid aluminum salt and the solid water-soluble polymer are added to the water-containing sludge at the same time or, more preferably, both of the powders of them are premixed to be used as a first agent. In a case that the aluminum salt is used in a solution state, the aluminum salt can be added using aluminum chloride high in basicity simultaneously with adding the water-soluble polymer to the water-containing sludge. A hydraulic substance can be added as a second agent over two stages by simultaneously adding the first agent composed of the anionic water-soluble polymer and the aluminum salt. By this constitution, the solidifying work in the solidifying process can be simplified.