A measuring method of bromate ion concentration includes a first fluorescence intensity measuring process including a process of passing hydrochloric acid through an anion exchanger to elute bromate ions adsorbed to the anion exchanger into the hydrochloric acid and a process of measuring the fluorescence intensity of the hydrochloric acid passed through the anion exchanger, a second fluorescence intensity measuring process including a process of passing a hydrochloric acid solution containing a fluorescent substance whose fluorescence intensity changes due to coexistence of bromate ions through an anion exchanger to elute bromate ions adsorbed to the anion exchanger into the hydrochloric acid solution and a process of measuring the fluorescence intensity of the hydrochloric acid solution passed through the anion exchanger, and a calculation process determining the bromate ion concentration in the water sample by using the difference between the fluorescence intensities of the hydrochloric acid solution and the hydrochloric acid.