To easily obtain a reaction curve suitable for curve fitting.

This measuring instrument prepares an initial concentration of analyte solution at first, and conducts measurement processing for feeding the analyte solution to an SPR sensor to measure a reaction situation with a ligand. A measured data obtained by the measurement processing is compared with a preliminarily stored reference value to determine the propriety of the measured data. When the measured data is determined to be proper, the curve fitting is carried out, based on the measured data to find a reaction rate constant. When the measured data is determined to be improper, a concentration of the analyte solution used in the next measurement processing is determined, based on the measured data. The determined concentration of the analyte solution is prepared to conduct the measurement processing by the solution. The measuring instrument repeats such procedures until obtaining the proper measured data.