A method for measuring bromate ion is provided that provides high-sensitivity measurement results more simply and more quickly than conventional bromate ion measurement methods. A fluorescent substance that is quenched by coexistence with bromate ions is added to a sample 130 and the fluorescence intensity of the fluorescent substance after quenching is measured, the measured fluorescence intensity being subtracted from the fluorescence intensity of a standard sample containing no bromate ions to calculated the fluorescence intensity difference. The bromate ion concentration is calculated from the calculated fluorescence intensity difference, using a pre-determined calibration line between the fluorescence intensity difference and the bromate ion concentration.