OXIDIZING GAS SENSOR

An oxidizing gas sensor excellent in sensitivity to an oxidizing gas, comprising: a substrate 2 having a open space portion 21 in which the thickness is reduced; an insulating layer 3 formed on the surface of the substrate so as to cover the open space portion 21; a heating element 5 formed in the insulating layer 3 at a position corresponding to the open space portion 21; a gas sensing element 4 formed on the surface of the insulating layer 3 at a position corresponding to the heating element 5; and a pair of electrodes 6 disposed to contact the gas sensing element 4. The gas sensing element 4 includes a sensitive layer 41 composed mainly of tin oxide, and a gas-permeable catalytic portion 42 formed mainly of gold at least on a portion of the surface of the sensitive layer and having gas permeability. The catalytic portion preferably has a mass thickness of 0.008 to 170 nm. The structure of the sensitive layer preferably is an agglomerate of tin oxide columnar crystal grains, and the crystal grains preferably have an average minor axis length of 7 to 65 nm.