[Object] To provide a gas sensor having a self-diagnostic function with a simplified structure.[Means of Realizing the Object] A gas sensor (30) includes two field-effect transistors and gate electrodes on gate insulation films (24) of the two field-effect transistors to detect gas using the gate electrodes. The gas sensor (30) includes a first gate electrode (5), a second gate electrode (6), and voltage applying means. The first gate electrode (5) is provided on one of the field-effect transistors. The second gate electrode (6) is provided on another one of the field-effect transistors. The voltage applying means is for, with the first gate electrode (5) and the second gate electrode (6) coupled to one another by wiring, applying thereto one of a direct-current voltage and an alternating-current voltage having a same potential or a constant voltage difference. The first gate electrode (5) and the second gate electrode (6) are made of different metals. The one field-effect transistor and the other field-effect transistor have approximately the same structures.