In the field of standard material development, there is a demand for a primary method of measurement, which is based on calculations relying on applicable physical and chemical laws, and whose results are directly related to the International System of Units. Though titrimetry has a potential to be a primary method, it is required to detect an accurate equivalence point with low uncertainty depending on the titration system. The equivalence point of a chelatometric titration was determined by analyzing a titration curve with a theoretical equation, in which equilibrium among metal ions, metal chelates and an indicator were taken into account. While absorbances at a single wavelength during a titration are observed with usual photometric titrators, absorption spectra can be observed with a spectrophotometric titrator. Using our method, it became possible to determine a much more precise equivalence point by analyzing the three-dimensional titration data.