PROBLEM TO BE SOLVED: To prevent small particles including a radioactive material from mixing with reusable soil having large particles.SOLUTION: A contaminated soil treatment apparatus 1 for separating small particles PS including a contaminant from a contaminated soil CS comprises: a cleaning device 2 for cleaning the contaminated soil CS; a classifier 3 for separating a primary contaminated residual water CW1 including the small particles PS out of the soil particles and large particles PB larger than the small particles PS out of the soil particles from the contaminated soil CS; a solid-liquid separator 5 for separating the large particles PB exhausted from the classifier 3 and a secondary contaminated residual water CW2 associated with the large particles PB and including the small particles PS; and a particle removal device 4 for removing the small particles PS from the primary contaminated residual water CW1 exhausted from the classifier 3 and the secondary contaminated residual water CW2 exhausted fro
