To efficiently perform an insolubilization treatment of a harmful substance, which is contained in polluted soil subjected to back filling.

In the treatment part conveyor in the treatment means D arranged on a base stand B subject the polluted soil from a supply part S to insolubilization treatment to discharge the treated polluted soil outside of the base stand B, at a region where the endless belt 42, which constitutes the treatment part conveyor 4 to be rotationally driven by an approximate drive source 43, is arranged above a treatment mechanism 6, each of both side parts of the endless belt 42 in an advance direction is made higher than the central part in the advance direction of the endless belt 42.