Method of detecting carbon monoxide gas at temperatures of 100.degree. C. or less by using a detecting element containing stannic oxide as the main component, which element is prepared by mixing stannic oxide, antimony trioxide and platinum in atomic ratios of Sb/Sn = at least 0.005, Pt/Sn = at least 0.004 and Sb/Pt = at least 0.005, and sintering said mixture at a temperature of at least 625.degree. C.