To provide a method for treating a contaminant in soil, by which the contaminated soil can be decontaminated efficiently without causing the contaminant to leak to the outside while saving expenditures and labor.

The contaminant and an organic carbon compound contained in the contaminated soil are vaporized by heating the soil. The gas containing the vaporized contaminant and organic carbon compound is heated in an oxygen- lowered atmosphere so that the organic carbon compound is carbonized to produce carbon particles. The produced carbon particles are collected and the contaminant adsorbed on the collected carbon particles is recovered. Otherwise, the contaminant contained in the soil is vaporized by heating the soil. The vaporized contaminant is cooled to obtain a condensate. The contaminant is recovered from the obtained condensate, namely, the condensate is separated into a hydrophilic phase and an oleophilic phase, the dust contained in the hydrophilic phase is separated and the contaminant is removed from the hydrophilic phase by making the hydrophilic phase pass through a separation membrane.