To provide a system and a method for treating contaminated soil, by which a large quantity of earth and sand to be treated can be treated continuously at a high speed by improving the decontaminating efficiency of a volatile organic compound contained in the earth and sand to be treated.

The system is provided with a self-traveling contaminated soil treating machine 136 for mixing the earth and sand contaminated with the volatile organic compound with an additive for generating heat by reacting with water content, a treating unit 201 arranged in a tent 94 for promoting the volatilization of the volatile organic compound contained in the additive- mixed earth and sand from the machine 136, a volatilization promoting means 200 having a discharge conveyer 202 for discharging the additive-mixed earth and sand volatilization-promoted by the unit 201, a volatile organic compound decomposing unit 80 for decomposing the volatile organic compound, and a gas sucking pipeline 96 for collecting the volatile organic compound volatilized in the tent 94 and sucking the collected volatile organic compound into the unit 80.