Oil contamination of the soil is one of the most serious environmental hazards especially in industrial areas over the world. We carried out an experimental investigation into the treatment of oil-contaminated soil to improve performance characteristics of the pilot plants which adopt washing treatment techniques. This improvement has been successfully achieved by micro bubble injection applied in separation processes. Especially, oil separation from high concentration oil-in-water emulsion in drainage has been extensively enhanced and, in fact, our results indicated that almost 70~80% oil particles have been effectively and successfully removed by flotation due to micro bubble injection. We also carried out visual observations of microbubble-oil particle/oil-contaminated solid particle interaction to discuss on the separation mechanisms. We have confirmed that flotation enhanced by excellent physical absorption characteristics and very low slip velocity of micro bubbles is a primary mechanism in oil separation processes.