Abstract: This invention provides a reducing gas sensor which has a gas sensitive element composed of, as a main constituent, .gamma.-ferric oxide (.gamma.-Fe.sub.2 O.sub.3) and, as an additive, at least one compound in the group of WO.sub.3, MoO.sub.3, SiO.sub.2, GeO.sub.2, Li.sub.2 O, Na.sub.2 O, K.sub.2 O, CaO, SrO, BaO, Eu.sub.2 O.sub.3, La.sub.2 O.sub.3, CeO.sub.2, Al.sub.2 O.sub.3, Ga.sub.2 O.sub.3, In.sub.2 O.sub.3, SnO.sub.2, TiO.sub.2, MgO, NiO, ZnO, Mn.sub.2 O.sub.3 and Cr.sub.2 O.sub.3, a pair of electrodes and a heater element. The .gamma.-ferric oxide is in the form of a thin film, a plate or a thin surface layer on a sintered plate of .gamma.-ferric oxide. The gas sensor of the invention is much improved in the stability during use and also it has a high sensitivity.

Abstract: This invention provides a reducing gas sensor which has a gas sensitive element of .gamma.-ferric oxide (.gamma.-Fe.sub.2 O.sub.3), a pair of electrodes and a heater element. The .gamma.-ferric oxide is in the form of a thin film, a plate or a thin surface layer on a sintered plate of .alpha.-ferric oxide. This invention also provides a method of producing the reducing gas sensor, which includes the steps of firing .gamma.-Fe.sub.2 O.sub.3, .gamma.-FeOOH or Fe.sub.3 O.sub.4 powder on a substrate and applying thereto a pair of electrodes and a heater element; or sintering alpha-ferric oxide (.alpha.-Fe.sub.2 O.sub.3) and reducing it to magnetite and oxidizing it at the surface thereof to .gamma.-ferric oxide as a gas sensitive element. The sensitivity (R.sub.A /R.sub.G) of the gas sensitive element of .gamma.-ferric oxide is 10 to 130.degree. at 270.degree. C in an atmosphere containing 0.1 to 1 percent by volume propane gas, and the reducing gas sensor has high sensitivity and stability.