PURPOSE: To make it possible to measure the small light absorption quantity at a high speed with high precision by measuring the light absorption quantity directly and automatically considering the characteristic intensity difference between both a measurement and a reference light beam.

CONSTITUTION: The measuring light beam M with wavelength λM and basic intensity IMa and the reference light beam R with wavelength λR and basic intensity IR are detected DE almost at the same time, and analog common collecting generation DE of the measuring signal UMa and basic signal URa in the form of difference generation D is performed. Consequently, the analog indication A of the intensity difference ΔI between the both as a direct measured light absorption quantity is made. An optical zero adjustment AN for it is made and the characteristic intensity difference ΔIn which attenuates to zero is automatically considered B. Then the beams M and R are transmitted through a substance MA to be measured which is inserted into a beam path S and the beam M is absorbed by the substance MA, but the beam R is not absorbed much and its adjusted intensity IRb decreases to IMb larger than transmitted light intensity IMc. Signals UMc and URc are generated by following detection DE and the difference between those signals is generated D automatically in consideration of the difference ΔIn and regarded as a direct measured value to obtain light absorption quantity information LA on the substance MA fast with high precision.