Provided are a method and system for measuring hydrogen peroxide concentration in sample water collected from a prescribed position in a water treatment process and includes: collecting the sample water; measuring a concentration of a dissolved oxygen in the sample water or a treated water obtained by treating the sample water with the hydrogen peroxide decomposing device by first and second dissolved oxygen concentration measuring analyzers to obtain a corrected value that is a difference between the two dissolved oxygen concentration values; measuring the concentration of the dissolved oxygen in the sample and treated water by the first and second dissolved oxygen concentration measuring analyzers, respectively, and obtaining a measured value that is a difference between the dissolved oxygen concentration values; and calculating a corrected concentration of hydrogen peroxide from the measured value obtained during the measured value obtaining and the corrected value obtained during the corrected value obtaining.