The analysis conditions for measuring trace concentration of Cr(VI) based on the ion chromatography-inductively coupled plasma-mass spectrometry (IC-ICP-MS) method was examined. By optimizing the analysis conditions, the measurement of 0.02 μg-Cr(VI)/L was achieved with a high degree of accuracy. The behavior of Cr(VI) through the water treatment process in Osaka city was investigated and it was determined that there was a tendency that the Cr(VI) concentration slightly fluctuated throughout the water treatment process with the highest concentration being observed in finished water. The Cr(VI) concentration at the outlet of Kunijima purification plant was less than 0.02–0.13 μg/L, while water taps where retention time from the plant was 4–15 h, were less than 0.02–0.10 μg/L. Also, it was found that Cr(III) was oxidized to Cr(VI) by sodium hypochlorite and ozone being used in the water treatment process. Based on this finding, the oxidation reaction of Cr(III) to Cr(VI) was examined. When sodium hypochlorite was added to granular activated carbon (GAC)-treated water and purified water containing Cr(III), the oxidation of Cr(III) to Cr(VI) proceeded and the Cr(VI) production in GAC-treated water was relatively higher. In addition, it was determined that the Cr(III) oxidation reaction rate depended on water temperature and it was faster when the water temperature was higher.