To provide a method for treating volatile matter in contaminated soil in a stock yard by which the volatile matter in the contaminated soil spread in the stock yard can be safely and economically treated.

A ventilation fan 3 driven by a variable speed motor 4 is arranged in the upper part of the stock yard 1 where the volatile contaminant-containing contaminated soil is spread and a gas treatment unit 6 for making a volatile contaminant harmless is arranged at the end of a ventilation duct 5 of the ventilation fan 3. A concentration meter 7 for measuring the concentration of the volatile matter which is volatilized from the contaminated soil and retained in the stock yard is arranged in the stock yard 1. The concentration of the volatile matter in the stock yard is measured by the concentration meter 7. When the measured concentration is high, the amount of the air to be ventilated is increased by rotating the ventilation fan 3 at high speed so that explosion does not occur. When the measured concentration is low, the power consumption of the ventilation fan is saved by rotating the ventilation fan 3 at low speed.