To provide a method for decontaminating soil and groundwater which enhances the feed of oxygen into the ground.

This method is for cleaning the soil and the groundwater by improving the feed of oxygen into a saturated layer S by injecting or penetrating a liquid containing fine air bubbles into the saturated layer S. When the feed of oxygen is improved, the activity of microorganisms in the soil is activated and the function of cleaning by microorganisms is improved, and contaminated soil and/or contaminated groundwater are efficiently cleaned by utilizing the microorganisms. The contaminated soil is washed and cleaned by such a flow of the liquid containing fine air bubbles that is generated in the ground by penetrating the liquid containing fine air bubbles into the groundwater.