PURPOSE: To prevent or lessen the elution of selenium from soil contaminated with selenium by adjusting the pH of contaminated soil contg. the selenium to a specific value.

CONSTITUTION: This insolubilization treatment method for the selenium contaminated soil comprises first putting a pH controller supplied from a tank 2 into a mixer 1 and stirring and mixing this controller. Controllers prepd. by properly diluting, for example, hydrochloric acid or sulfuric acid at need are used as the pH controller. Next, this soln. is put together with the contaminated soil contg. the selenium into the soil mixer 3 and is sufficiently stirred and mixed to adjust pH which is an index to indicate the concn. of the hydrogen ions in the soil to ≤6, more preferably to ≤5.5. As a result, the elution amt. of the selenium from the polluted soil is lessened to about 1/4 the elution rate from the original contaminated soil. The elution rate of the selenium from the original contaminated soil is decreased to ≤1/10 and is thus completely suppressed particularly if the pH is adjusted to ≤6 by adding an iron oxide to the contaminated soil.