PURPOSE: To safely and efficiently treat contaminated soil contg. mercury, etc., by accumulating the contaminated soil in a space surrounded with a water- impermeable wall, supplying an aq. sodium sulfide soln., discharging the soln. from a pipeline provided on the space bottom, adding iron sulfate and then sprinkling the soln. over the contaminated soil for the second time.

CONSTITUTION: The contaminated soil contg. at least one kind selected from mercury, lead, cadmium, zinc, manganese, etc., is treated as follows. Namely, Na2S and water are supplied to a tank 1 and agitated 2 to produce an aq. Na2S soln., and the soln. is sprinkled 3 over the surface of the contaminated soil in a contaminated soil treating plant 20 to fill the cavity of the soil. The soln. is discharged from a pipe 5 a specified time after sprinkling by a pump and stored in a tank 11. Ferrous sulfate is added to the stored soln. and agitated to allow the excess Na2S and S2- to react with Fe2+. The soln. in the tank 11 is then supplied to a sprinkling pipe 3 and sprinkled over the contaminated soil. Heavy metals including mercury are removed in this way.