PURPOSE: To capture Cl- ions in the water flowing in a water distributing copper pipe in a room to decrease the concn. of the Cl- ions in the used water and to prevent the pitting corrosion of the water distributing copper pipe by providing an inside pipe having plural through-holes to form the room to the inside of a copper pipe insulated from the water distributing copper pipe.

CONSTITUTION: The inside pipe 1 (insulating pipe) having the plural through-holes is provided to the inside of the copper pipe 3 insulated from the water distributing copper pipe 10 and the room 5 is formed of the flanges provided at both ends of the inside pipe 1 and the inside peripheral face of the copper pipe 3. The cathode of a DC power source 9 is connected to the copper pipe 10 and the anode is connected to the copper pipe 3. The water fills in the room 5 as well when the water flows to the copper pipe 10. The copper dissolves into the room 5 and the concn. of the Cu ions in the room 5 increases when electric current is passed from the power source 9. The Cl- ions in the flow on the outside of the room 5 enters the room 5 by passing the through-holes at the same instant. The Cl- ions in the used water are captured into the room 5 and the Cl- ions which are the cause for the pitting corrosion of the copper pipe 10 are decreased according to the above-mentioned mechanism.