Abstract: An apparatus for treating waste water such as sewage in which at least one bundle of a number of straight tubes is submerged in waste water to be treated, said bundle being particularly provided with a plurality of crossing passages extending across the bundle which passages are arranged intermittently along the bundle with certain intervals therebetween so that no Poiseuille flow is substantially formed in each of the tubes. The waste water is flowed through the bundle of straight tubes in a direction along the bundle. In the bundle of straight tubes, organic substances in the waste water are removed due to decomposition caused by microorganisms which adhere to the inner surface of each tube. Owing to the presence of the crossing passages, removal of the organic substances proceeds effectively and promptly due to uniform flow distribution across the bundle, a disturbance of flow given to the water, propagation of various kinds of microorganisms and the like.