A gas sensor is described which includes a layer of a sensitive material formed on an electric insulating substrate and spaced electrodes electrically connected to the layer. The layer is formed of a porous film of a uniform mixture which contains a p-type compound oxide semiconductor with a perovskite type of crystal structure as the major ingredient and one or more of vanadium, niobium, tantalum and/or compounds thereof as minor ingredients. The minor ingredients are contained in the layer in an amount of 0.01 to 5% by weight, based on the weight of the p-type compound oxide semiconductor, and are incorporated into the layer by diffusing them into the layer. The gas sensor exhibits a small change with time and a reduced tailing effect attendant on variations in the gas combustion. With this, it is possible to effect measurements, detection and control with a high reliability.