A total organic carbon measuring instrument including a measuring unit composed of, integrated together, organic substance oxidation part (24) and carbon dioxide separation part (20) and conductivity measuring part (34), control unit (40) and data processing unit (41). In order to enhance the accuracy of conductivity measurement, the control unit (40) is constructed so as to stop feeding of a sample water at the time of oxidation of organic substance and carry out feeding of the sample water at the time of sample water moving to the organic substance oxidation decomposition part (20) and carbon dioxide separation part (24). The data processing unit (41) is constructed so as to measure the total organic carbon concentration on the basis of conductivity at the time of arriving of sample water irradiated with ultraviolet rays of which relative intensity is a given value or higher at the carbon dioxide separation part (24).