Abstract: An apparatus for biologically treating lipid-containing waste water comprising a waste water introducer, a biological treatment constituted by a carrier holding a fixed lipid-assimilating, and a discharge for discharging the treated waste water. The carrier has a shape of string, lace, mat, sheet, fiber, sponge or film which facilitates the lipid-assimilating yeast to cling thereto. The apparatus may also be provided with backwashing for the biological treatment carrier, and an aerator for aerating the waste water. It is possible to directly treat lipid-containing waste water without producing any harmful matter and to efficiently reduce the concentration of the lipid (n-hexane extract) to not more than 30 mg/1. Furthermore, the maintenance of the apparatus is easy.