PURPOSE: To prevent the generation of reddish water and to efficiently remove red rust by projecting UV rays on the supply water from an electrodeless lamp not in contact with the supply water during the circulation of the supply water after the sterilization and decomposition stage by ozone.

CONSTITUTION: When the generation of red rust in the city water or sewage water in a high-rise building is prevented, ozone is brought into contact with the bacteria and body fluid layer from the bacteria adhered to the walls of the water supply pipe 3 and water distributing pipe 5 to sterilize and decompose the bacteria, etc., and the red rust is partially removed. The supply water after the stage is circulated through the water supply pipe 3 and distributing pipe 5 to physically remove the remaining red rust, the UV rays from microwave- stimulated electrodeless lamp is projected on the transparent water supplying and distributing pipe arranged in a water sterilizing device 4, the water is further sterilized, and the red rust is removed.