Bromate ion concentration in groundwater was determined for 572 non chlorinated wells and 16 springs in Tama district, Tokyo. Bromate ions were detected in 30.9% of shallow wells, 21.6% of deep wells and 56.2% of springs. Their concentrations ranged from 0.1 to 5.6 μg·l^-1. In five shallow wells, bromate ion concentrations of 2.1 μg·l^-1 or higher were detected. We also determined bromate ion concentration in chlorinated groundwater samples (from 85 wells and 86 tap water supplies from groundwater). Bromate ions were detected in 38.8% of the wells and 74.4% of the tap water samples. Their concentrations ranged from 0.1 to 57.7 μg·l^-1 (well water) and 0.2 to 13.8 μg·l^-1 (tap water). A follow-up survey of bromate ions in seven wells (4 non chlorinated and 3 chlorinated) was performed. Bromate ion concentration in non chlorinated well water hardly changed during the survey period. Of the 3 chlorinated wells, 2 showed no rechlorination in the second sampling; bromate ion concentrations in these two wells decreased.