To prevent the reverse flow of a material by eliminating a positive pressure generated in a treatment tank in an oil separation apparatus for an oil-containing substance.

An ejector 62 is mounted to a distal end of an injection nozzle 61 for injecting supersonic stream steam of high temperature and high pressure. The oil separation apparatus is constituted such that the material, i.e., the oil-containing substance is sucked from a suction port 63 of the ejector 62 and is led out from a discharge port of the ejector 62 into the treatment tank 6. Thereby, the inside of the ejector 62, i.e., a part of the material injection port becomes not the positive pressure but the negative pressure by the injection steam of supersonic stream. Thereby, the reverse flow of the material is not generated and control of material feeding is also accurately performed.