An electrochemical corrosion potential sensor has a sensor unit, a lead wire and a quasi-reference electrode. A sensor unit includes a tube-shaped insulator, a tube-shaped metal casing joined to an end portion of the insulator, and a Pt electrode joined to another end portion of the insulator. A lead wire connected to the Pt electrode passes through the insulator and the metal casing. The quasi-reference electrode disposed in the metal casing is made of a less noble metal and electrically connected with the lead wire.Since an electrochemical corrosion potential sensor has the quasi-reference electrode, the measurement of the corrosion potential of a structural member of a nuclear power plant and an abnormality occurrence (water intrusion) can be accurately detected during the operation of a nuclear power plant.