Sea water leak detection method and device

In a method of detecting sea water leakage by detecting it based on electroconductivity or electric resistance of condensates taken out of a condensator of a power plant, the condensates are degassed to remove gaseous carbon dioxide dissolved in the condensate, before the detection for the electroconductivity of the condensates. Alternatively, a degassing means for removing gaseous carbon dioxide dissolved in the condensate is disposed to a prestage of the condensate electroconductivity measuring means. The degassing means comprises a membrane-degassing device, and the membrane-degassing device comprises a hollow thread membrane for passing through the condensate and permeating gaseous carbon dioxide in the condensates and an evacuating device to form a pressure reduced space on the side opposite to the water passing side of the hollow thread membrane, and allows the dissolved gaseous carbon dioxide in the condensates to permeate on the side of the pressure reduced space by way of the hollow thread membrane. Erroneous operation of the condensator due to vacuum breakage is prevented thereby capable of reliably detecting the leakage of sea water, and the power plant can be restarted rapidly with no time loss. (N.H.)