To provide a method and an apparatus in which the separation of oil and water is performed at a high speed even in a small scale.

A liquid to be treated is separated into oil and water by making the oil contained in the liquid float with bubbles supplied to the liquid stored in a treating vessel 11. Air is dissolved by an air supplying means in the liquid taken in from the bottom of the treating vessel 11, and the liquid is injected from a nozzle 33 installed at the bottom of the vessel 11 to be returned to the vessel 11, thereby realizing the circulation of the liquid. The liquid before treating is supplied to a circulation pipe for the liquid or to the bottom of the vessel 11. Air is sufficiently dissolved in the liquid by the air supplying means in the circulation pipe so that the bubbles may surface uniformly in the vessel.