From a screening test for the sensing properties of 26 single oxides, an In2O3 sensor element was found to exhibit rather high sensitivity to both CO and H-2 in wet air at 300 degrees C. In order to improve the selectivity to CO vs. H,, each of 33 metal oxides was added to In2O3. As a result, the addition of alkaline metal oxides was found to enhance markedly both sensitivity and selectivity to CO. Particularly, Rb2O (5 weight percent)-In2O3 element showed the highest CO selectivity. It could detect 500 to 4000 ppm CO in wet air at 300 degrees C satisfactorily.