The hydrogen-sensing property of a new type field-effect gas sensor device based on silicon was studied. The device had an FET structure with a Schottky gate and was fabricated from an n-on-p + Si substrate by a lift-off process. Variation in electric potential of a floating gate was used as a sensing signal, together with a change in drain–source voltage. The device can detect hydrogen in an oxygen atmosphere at room temperature. Both the voltage between the drain and source electrodes, V DS , and the voltage between the gate and source electrodes, V GS , change greatly under a constant current, because of an oxidation reaction of hydrogen. When the substrate is connected with the source electrode, the V GS has a unique characteristic with respect to a change in hydrogen concentration. V GS becomes highest at the hydrogen concentration where V DS changes most sharply. Water vapor prevents hydrogen detection. The behavior derives from both a change in distribution of channel resistance caused by a change in ambient gas and an interaction between two current components, drain to source and drain to substrate.