PURPOSE: To provide an ultraviolet-ray absorbance measuring method for process and its device capable of accurately and continuously measuring the ultraviolet- ray absorbance derived from the absorption of an organic matter even when dissolved ozone exists in the ozone-processed water.

CONSTITUTION: The water to be processed is ozone-processed in this process. The ozone-processed water is fed to the first measuring cell 12 as sample water, the light passing through the sample water from a light source is received by a light receiver, and the sum of the ultraviolet-ray absorbance derived from an organic matter and the ultraviolet-ray absorbance derived from dissolved ozone is measured by a converter/calculation section. The sample water finished with measurement is fed to an aeration tank 14 kept at the decompression state, the dissolved ozone is extracted as ozone gas by the diffusion of air, the de-ozone processed water is fed to the second measuring cell 13, and the ultraviolet-ray absorbance derived from only the organic matter is measured.