To provide an apparatus capable of efficiently and securely carrying out insolubilization treatment even if backfill soil is complexly contaminated with harmful substances.

The terminal end of a feed section conveyor 2 constituting the feed section S disposed on the base B is positioned above the starting end of a treatment section conveyor 4 constituting the treatment section D also disposed on the base B, with the feed section conveyor 2 and the treatment section conveyor 4 forming a L shape as viewed from above in the travel direction of contaminated soil. A treatment mechanism 5 constituting the treatment section D is positioned above the junction of the terminal end of the feed section conveyor 2 and the starting end of the treatment section conveyor 4. The treatment mechanism 5 is equipped with a chemical agent feed hopper 51 that allows feeding of a chemical agent and feeds the chemical agent to contaminated soil transferred from the feed section S.