The bluish water generated in hot water and cold water supply piping systems used the copper pipe was examined by the case analysis and the experiments. It is shown that the phenomena occur with easy when the pH is low, the density of the free carbonicacid is high, and the density of the dissolution salts are high. The phenomena of bluish water are divided into the blue dirt such as bath equipment for, dyeing laundry blue, and looking blue to the water of bathtub, etc. The following were clarified from the case analysis and the experimental results. The bath equipment becomes dirty in blue by the products such as the copper soap or copper hydroxycarbonate. It is coloring of the cloth with the copper soap to dye laundry in blue. The water of the bathtub looks blue because visible rays are selectively absorbed with water.
