A catalytic-combustion-type carbon monoxide gas sensor operable at a low temperature was devised by employing a Pt-loaded cerium oxide-zirconium oxide-tin oxide solid solution as a novel oxidizing catalyst. Since this catalyst can oxidize CO at low temperatures, the sensor with the Pt-loaded CeO2-ZrO2-SnO2 catalyst exhibits a smooth and reproducible response to CO gas at 70 degrees C, which is a significantly lower temperature than that required for the operation of sensors with the conventional CO oxidation catalysts Pt/Al2O3 or Pd/Al2O3.