A gas sensor includes a first electrode having a first main surface and a second main surface opposite to the first main surface; a second electrode having a third main surface facing the second main surface and a fourth main surface opposite to the third main surface; a metal oxide layer disposed between the first electrode and the second electrode, and being in contact with the second main surface and the third main surface; and an insulating film covering at least a part of the first electrode, a part of the second electrode, and at least a part of the metal oxide layer. At least a part of the fourth main surface is exposed to gas which contains a gas molecule including a hydrogen atom. A resistance value of the metal oxide layer decreases when the second electrode is in contact with the gas molecule.