Among the In2O3-based elements loaded with each of 12 metal oxides [0.5 weight percent (w/o) each], the Co2O4-added element was outstanding in sensitivity and selectiveness to CO at 250 degrees C. Remarkably the sensing properties of this element could be increased further by the addition of Au (0.04 w/o). The element thus obtained, Au (0.04 w/o)-Co (0.5 w/o)-In2O3, gave excellent characteristics in sensitivity to CO, selectivity to CO against H-2 and other selected gases, and responses rate at 250 degrees C, giving promise of being applied as a CO sensor for safety control of gas appliances.