To purify water in a tank by operating a water feeding pump arranged in the tank.

In the tank 112 which stores water, the water feeding pump 113 is arranged. In this case, the water feeding pump 113 is constituted of a hollow cylindrical section 117, an impeller 119, a suction port 120, a discharge port 121, and a discharging pipe 115. The outer periphery of the hollow cylindrical section 117 is covered with a water-purifying filter 118. The impeller 119 is arranged in the hollow cylindrical section 117. The suction port 120 is provided on the side wall of the hollow cylindrical section 117. The discharge port 121 is provided on the hollow cylindrical section. One end of the discharging pipe 115 is connected to the discharge port 121, and the other end of the discharging pipe 115 is projected to the outside of the tank 112. Then, the impeller 119 is rotated in the opposite direction from the rotation by which water is discharged. Thus, the water in the tank 112 circulates in the water purifying filter 118 and the tank 112, and is purified.