PURPOSE: To purify water during recirculation while appropriately aerating the same, by providing a water sucking pump to one side of a basin while providing a water discharge apparatus to the other side thereof to connect both of them by a water sending pipe, and packing a part of the water sending pipe with a water passing microorganism bed.

CONSTITUTION: A water sucking pump 2 is provided to one side of a relatively shallow basin 1 and a water discharge apparatus 10 is provided to the other side thereof. Water is sucked by the pump 2 through water sucking pipes 4, 4a, 4b sent to water purifying pipes 8, 9 from an emitting pipe 5 through water sending pipes 6, 7. The water purifying pipes 8, 9 are respectively packed with honeycomb cores and a microorganism is deposited to the surface of each cell of said honeycomb cores to treat BOD of water to be treated. Treated water further enters the water discharge apparatus 10 through water sending pipes 6a, 7a and sprinkled over the water surface through water sprinkling nozzles 11, 11a, 11b of the water discharge apparatus 10 to be sufficiently aerated. In this case, because the water purifying pipes 8, 9 are freely rotatable, the precipitation of solid substance to the honecomb cells can be prevented.