An extended-gate field-effect-transistor (FET) sensor with a gold-sensing electrode, to which a gold-thiol bond could easily be applied, was developed for DNA detection. Because the gold electrode is located in a different area from the FET, it can be operated without a light-shielding box by masking only the FET. However, when the FET sensor is used in an aqueous solution, fluctuation of the interface potential on the gold surface occurs, which results in decreased sensitivity. In DNA detection, 1 h or more was required to stabilize the drain current of the FET sensor after dipping it into the solution. To improve the sensitivity by reducing the fluctuation, we devised a measurement technique using a high-frequency voltage superimposed onto a reference electrode. With a superimposed high frequency voltage of over 1 kHz, the time required to stabilize the drain current of the FET sensor after dipping it in the solution was not only shortened to 5 min, but the fluctuation of the drain current was also reduced. As a result of applying this method, the FET sensor could successfully detect DNA hybridization and the extension reaction.