To reduce the amount of contaminated soil required to be carried out by efficiently classifying a small-diameter particle not more than silt having a high heavy metal contaminated concentration from soil when soil contaminated with heavy metals is decontaminated in situ.

A method comprises the steps of charging soil and water contaminated with the heavy metals in a fluidized bed type vessel 1 including a cylindrical part 2 and a conical part 3 provided with an introduction port 4 of high-pressure air at a lowermost part, maintaining a fluidized state by introducing the high-pressure air upward from the introduction port 4 of the high-pressure air, forming each layer of a gravel layer, a sand layer, a silt layer, a clay layer and a colloid layer in turn from the bottom of the vessel, after discontinuing introduction of the high-pressure air, discharging the small-diameter particle including the silt layer, the clay layer and the colloid layer from the top of the vessel, classifying soil of the small-diameter particle not more than silt, removing to recover soil of a large-diameter particle not less than sand, and turning soil of the large-diameter particle back to the existing foundation or the like of the contaminated soil.