An aeration-less water treatment system includes an anaerobic treatment tank which subjects sewage supplied by a pump to anaerobic treatment by causing the sewage to upflow, and an aerobic treatment tank which subjects anaerobically treated water to aerobic treatment by letting the anaerobically treated water to downflow by natural downflow. The aerobic treatment tank includes an aerobic filter bed section in which carriers to which aerobic microorganisms are adhered are arranged. The aerobic filter bed section subjects the anaerobically treated water to decomposition treatment by putting the anaerobic treatment water in contact with the aerobic microorganisms adhered to surfaces of the carriers. The aerobic filter bed section is provided with an air port which takes in air into under normal pressure, thereby activating the aerobic microorganisms.