A gas sensor wherein a first electrode whose surface is covered with an oxidizing catalyst, and a second electrode which is made of the same material as that of the first electrode and whose surface is exposed and can come into direct contact with a gas to-be-detected are disposed on a solid electrolyte which exhibits an oxygen ion conductivity.Since the gas sensor of this invention has the electrodes formed of the identical material, it has over a prior-art gas sensor the merits that secular changes are very little and that the operation is possible even at low temperatures. It can readily detect a combustible gas on the order of several tens p.p.m.