To sterilize and clean bacteria and suspended matters adhered to a hollow fiber membrane by tap water, by disposing the hollow fiber membrane and a dechlorination agent apart each other, and forming a second waterway reaching the hollow fiber membrane without passing through the dechlorination agent outside a first waterway, in which supplied water passes through the dechlorination agent and reaches the hollow fiber membrane.

A hollow fiber membrane 4 and a dechlorination agent 5 are disposed apart each other in a water purifier 1, and a first waterway 10 and a second waterway 11 are connected with a same raw water supply line 9 through a three-way valve 12. In the case of utilizing the above water purifier 1 for the purification of tap water at a home, tap water is not passed through the first waterway 10 connecting the raw water supply line 9 with a dechlorination agent 5 storage section or the dechlorination agent 5 storage section itself but through the second waterway 11 connected with a hollow fiber membrane zone 4 and inserted into a cartridge 2. Also, the tap water is flowed only in either of the waterways 10 or 11 by operating the three-way valve 12 provided at a branching point of the first waterway 10 and the second waterway 11 in the constitution of a system of the water purifier.