To provide a solidification agent and a solidification method which enable a sufficient solidification of waste soil having high fluidity, generated in civil engineering/construction works, wet sludge, such as muddy sediment (sludge) in water, and the like while preventing offensive odors by one-stage mixing process.

The solidification agent for wet sludge treatment consists of inorganic particles, preferably synthetic silica, and a water-soluble polymer. The solidification agent for wet sludge treatment contains 40-95 wt% inorganic particles and 5-60 wt% water-soluble polymer. The inorganic particles are made to have a given BET specific surface area, thereby the solidification agent has a sufficient offensive odor reducing effect and solidification performance even when the water-soluble polymer and the inorganic particles are mixed beforehand.