To reliably discharge wastewater in a water tank within a short time in a reproducing mode.

In this apparatus, a conductive first filter 14 is installed in a water tank 10 at a gap from the circumferential wall 10a of the tank and a second filter 15 having water flow holes smaller than that of the first filter 14 is installed in the inside of the first filter 14 and the apparatus is so constituted as to have a water purifying mode to capture microorganism existing in raw water and a reproducing mode to discharge the microorganism captured by the first and the second filters 14, 15 by discharging raw water lingering in the water tank 10. In this case, an air introducing route 20 penetrating the circumferential wall 10a of the water tank 10 and communicated with the gap 16 is formed and at the same time an air introducing valve 22a to open and close the route 20 is installed in the air introducing route 20. Consequently, at the time of reproducing mode, atmospheric pressure is applied to the water discharge side through the gap 16 and water in the water tank 10 is discharged within a short time.