PURPOSE: To eliminate necessity for providing a water purifier in each water supply facility of dwelling houses and provide possibility of supplying purified water by installing a water purifying device at a main pipe which supplies service water from a public main to the water supply facilities.

CONSTITUTION: From a public main PT a branch pipe 8 is extended to each water supply facility 10 of a house 2 via a main pipe 6 furnished in the site 4 of the house 2, and a main valve 14 and water meter 16 are mounted in the upstream of the main pipe 6 while a water purifying device 20 is installed in the downstream. When the service water from the public main PT to each facility 10 is flowing in the main pipe 6, the purifying device 20 mounted thereon purifies the water in a lump, which eliminates necessity for provision of individual purifiers on the water supply facilities.