method for optically analyzing a specimen by using two light beams comprises steps of initiating automatically wavelength scanning operation when a predetermined time has elapsed after the switching - on of a power source , sampling a ratio between a reference light intensity and a specimen light intensity during the wavelength scanning operation when no specimen to be measured is placed in the sample cell , and storing signals derived from the sampling operation in a volatile memory . when the specimen is to be measured or analyzed , the measure signal representative of ratio between the reference light intensity and the specimen transmitting light intensity which are produced or the wavelength scanning operation proceeds is corrected by the corresponding signal read out from the volatile memory in accordance with the wavelength of the measure signal , and the corrected signals are processed as the error - free measure signal .