PURPOSE: To enable highly accurate analysis of the concentration of a specified substance addressing emergency analysis quickly by performing a photometry of a reaction liquid at a plurality of specified reaction times separately to determine concentration values based on a plurality of photometric data and a corresponding calibration curve generated previously and further a true concentration value based on the plurality of concentration values.

CONSTITUTION: Absorbance of a sample to be measured at reaction times TA and TB is analyzed in terms of reaction component and error component to determine a concentration value. The error component of the absorbance is determined based on the concentration value on condition that the absorbance of the reaction component is constant in ratio in terms of reaction time and the error component is uniform regardless of the reaction time. Then, the error component is corrected from any one of the absorbances measured to determine an absorbance attributed to reaction and a true concentration value is determined based on a calibration curve corresponding to the absorbance. Thus, the error attributed to the fouling of a reaction container is corrected thereby always enabling analyzing of the concentration of a specified substance at a high accuracy and also addressing of a demand for emergency analysis quickly, eventually achieving higher processing efficiency.