<p>[PROBLEMS] To use in a carbon dioxide separation part a gas-permeable membrane capable of maintaining high-speed determination and to diminish the influence of interfering ingredients. [MEANS FOR SOLVING PROBLEMS] An apparatus which comprises: an organic-substance oxidation part in which organic substances contained in a sample water supplied are oxidized into carbon dioxide; a carbon dioxide separation part in which the carbon dioxide contained in the sample water treated with the organic-substance oxidation part is caused to permeate and come into a test water; and a conductivity measurement part in which the conductivity of the test water sent from the carbon dioxide separation part is measured. The carbon dioxide separation part comprises: a sample water channel in which the sample water treated with the organic-substance oxidation part flows; an intermediate water part in which an intermediate water having a higher pH value in a neutral region than the sample water flowing through the sample water channel is present; and a test water channel through which a test water comprising deionized water flows. The sample water channel is in contact with the intermediate water part through a gas-permeable membrane, and the intermediate water part is in contact with the test water channel through a gas-permeable membrane.</p>
