PURPOSE: To enable to perform a measrement of an accurate O3 concentration without being interfered with a subaqueous coexisting component, by a method wherein a gaseous matter, passing a aeration cylinder, in which water containing a specified amount of O3 is brought into contact with a specified discharge of a cleaning gaseous matter, is fed to an optical cell, and O3 of an air phase is measured directly by an ultrasonic absorbance method.

CONSTITUTION: Water to be tested is fed at a specified discharge to an aeration cylinder 1 by a quantitative pump 1, and an aeration cleaning gaseous matter (air will do) is fed as air bubbles from the bottom part of the cylinder 12 through a compressor 13, a filter 14, a pressure reducing valve 15, and an air diffusing pipe 16. Oxidizing chlorine (ClO-, HClO), NO3-, Fe+3 and the like in water to be tested remain in water, and air containing O3 is fed to an optical cell 5. Ultrasonic-rays from a light source 6, i.e., low-pressure mercury lamps, are projected to the cell 5 through a filter 7, and an absorbance by O3 having a wavelength of 265nm is measured by a detector to display it in a recorder. This causes O3 concentration to be accurately measured directly without being interfered with an impeding component in water to be tested.