To efficiently sterilize execute strization without impairing the purifying performance in a water purifier for performing biological purification.

In this water purifier, in order to separate an ozone sterilizing tank 7 taking a reaction time and being in danger of damaging the bacterial used in a biological purifying tank 10 from the biological purifying tank 10, a flow path 4 circulating in the device is branched to an ozone sterilization tank side flow path 4a and a biological purifying tank side flow path 4b just behind a water sucking port 1, and sterilization and purification are performed with respective tanks 7, 10, and then the ozone sterilization tank side flow path 4a and the biological purification tank side flow path 4b are again joined to make the flow path 4. Thus, the possibility that the water mixed with ozone in the ozone sterilization tank 7 flows into the biological purification tank 10 to kill the bacteria is eliminated, then an intrinsic biological purification effect can thoroughly be exhibited.