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 is caused to permeate and come into a measurement water; and a conductivity measurement part in which the conductivity of the measurement water is measured. The carbon dioxide separation part comprises: a sample water channel in which sample water flows; an intermediate water part in which intermediate water having a higher pH value in a neutral region than the sample water is present; and a measurement water channel through which the measurement 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 measurement water channel through a gas-permeable membrane.