To accurately measure a trace organic matter in water such as extrapure water by oxidizing and decomposing the organic matter in sample water and measuring ions in the oxidized and decomposed water treated.

A sample water 1 is made to flow through an ultraviolet oxidizing device 3 by an oxidizing and decomposing portion, organic matter in the sample water 1 is decomposed with irradiation of ultraviolet ryas, and the treated water is fed to a measuring portion. In the measuring portion, the treated water fed from a treated water bottle 4 is concentrated by a concentration column 5, and decomposed water is discharged out of a system. The concentrated water is injected into a separation column 8, and an eluent is injected from an eluant bottle 9 to develop and elute an ion component absorbed to the separation column 8. The eluent travels through a suppresser 7, and a detector 6 detects the ion component. In this ion component measurement, measurement for each ion component can be performed, therefore, a speak of only bicarbonate ions after removing the other inorganic ions can be detected, and based on the area an accurate organic concentration as a carbon concentration can be measured up to ppb order.