In an automatic analyzer which includes a reaction container which contains reaction solution therein, a light source which emit light to be transmitted through the reaction solution, a spectral detector which measures the light transmitted through the reaction solution, a memory which stores light measurement data measured by the spectral detector and a CPU which calculates the light measurement data stored in the memory to obtain a light intensity, wherein the spectral detector measures the light over an entirety of an area from one end to the other end of the reaction container at a portion where the reaction solution reserves, the memory stores the light measurement data measured by the spectral detector, and light measurement data in an area where the reaction solution exists is obtained from the memory to calculate a light intensity.