Total hardness and concentrations of calcium and magnesium, and partly detection of nitrous ion and ammonium ion in tap water, well water, mineral water drinks, etc., in the summer of 1987 in Tokyo and the around areas, and the survey of taste for mineral water drinks were investigated. The average hardness in tap water and well water were about 69 ppm and 96 ppm (as the amount of CaCO₃), respectively, and the average concentrations of calcium and magnesium were about 19 ppm and 5 ppm, respectively, in tap water and about 22 ppm and 8 ppm, respectively, in well water. Tap water stored in a water tank of a building showed rather low hardness, compared with tap water unstored in the tank. Nitrous ion or ammonium ion was detected in 9 out of 13 the stored tap water samples and more than halves of well water and natural water samples. It indicates that tap water in the water tank and ground water were contaminated. The order of good taste for mineral water sample (total hardness 27-650 ppm) was almost the same as that of lowness of water hardness.