In this study, we investigated the formation condition of bromate ions following chlorination of purified water or of well water containing bromide ions. The formation of bromate ions occurred at room temperature but increased with temperature or pH. In the presence of ultraviolet rays, bromate ions formed rapidly and in high concentrations, but could be almost completely controlled by shading during chlorination. Light irradiation of chlorine-treated well water containing bromide ions also led to formation of bromate ions at concentrations ranging from ND to 60.8 μg·l^-1, but concentrations differed greatly between different well water samples. This difference was thought to be the result of differing concentrations of organic matter contained within the different well water samples. We suggest that bromate ion formation is suppressed by the high chlorine concentration used for trihalomethane formation when the organic matter concentration is high, and that these two formation reactions competed for chlorine. There is also a possibility that some organic matter, such as humic materials, directly controls the formation reaction of the bromate ion.