Abstract: It relates to a treatment of oil-containing waste water using a membrane biological reactor membrane biological reactor biological reactor (MBR), and it is to provide a new treatment apparatus which is capable of suppressing a reduction in biological treatment activity and suppressing an effect on a separation membrane. Provided is a membrane biological reactor having a biological reaction chamber and a membrane separation chamber, in which the it has a configuration that, within the biological reaction chamber, at least one partition is installed to have a first reaction chamber, a second reaction chamber, and if necessary, an additional reaction chamber so as to form an upflow and downflow flow path, an aeration device and a scum/oil skimmer are installed at least in the first reaction chamber, and a mixture liquid containing activated sludge is withdrawn from the membrane separation chamber and distributed and returned at least to the first reaction chamber and the second reaction chamber.