PURPOSE: To make installation and maintenance of a water purifying apparatus easy by jetting water under water above the surface of the water by a water pump, putting a filtering material in a water sucking route in the water of the water pump, and decomposing and converting organic substances in the water into inroganic substances by a biological membrane formed on the surface of the filtering material.

CONSTITUTION: Water is sucked through a suction inlet 5a of a water pump 5, supplied to a discharging pipe 21, a T-shape pipe 23, a connecting hose 25, and a circular pipe 17 and ejected by a water ejecting nozzle 16 set above the surface of the water. At the time of suction through the suction inlet 5a, water in the outside of a lower outer frame part 9 flows in to the space R2 in the inside of a lower inner frame part 11. At that time, water is led to a filtering material 12 and passes the filtering material, so that floating and suspended solid is absorbed and collected and at the same time organic substances are oxidized and decomposed by a biological membrane consisting of aerobic bacteria to convert the organic substances into inorganic ones. The resultant inorganic water is ejected from the water ejecting nozzle 16 and falls to the surface S of the water. During that time, the intake of oxygen in air is carried out.