This paper proposes a new corrosion index that can be effectively applied to tap water having low calcium hardness and high soluble silica (SiO2) concentration. Tap water at 70 different locations in Japan were sampled, and corrosivity for carbon steel dipping in flowing tap waters were evaluated. The average corrosion rate vavg was obtained by the weight loss method. The results showed that vavg was strongly dependent on the concentrations of SO42- and SiO2. The application of conventional corrosion indexes to Japanese tap water was examined with unsuccessful results. In view of these facts, a new corrosion index SS ratio = [SO42-] / [SiO2] was proposed. It was quantitatively demonstrated that vavg was directly proportional to the SS ratio. At the same time, the maximum localized corrosion rate vmax was determined by measuring the corrosion depth with a point micrometer. The results show that vmax could also be well evaluated by the SS ratio, and vmax was inversely proportional to the SS ratio.