To provide a method for preventing disadvantages that a water supply pipe 9 which supplies city water rusts due to oxygen, iron, chlorine or the like contained in the city water if city water is supplied to a condominium building, an office building or the like, this rust grows to a rust lump inside the water supply pipe, a slime-like rust is deposited on this rust lump and the rust flows as a red water, which causes a water leakage accident, a drop in water flow rate and a deterioration in function of the water supply pipe.

Since a constitution is so made that the slime-like rust or the like deposited on the rust lump generated inside the water supply pipe 9 are removed by pressurizing a compressed air and water into the pipe 9, and a water purifier 18 is incorporated between a water supply riser 8 and the pipe 9, the chlorine contained in the city water can be cut off by 100%.