This invention relates to an element which can detect selectively carbon monoxide from other reducing gases contained in air. The element has a sensitive response to carbon monoxide at a comparatively low temperature and not to other reducing gases. The element has a composition comprising stannic oxide as a base material, platinum black as a catalyst and, if necessary, clay, ferric oxide or glassy substances as a sintering agent. A starting substance for stannic oxide may be stannic oxide itself or any stannic salt which is changed into stannic oxide by baking. A starting substance for platinum black may be platinum black itself or any platinum salt which is changed into platinum black by baking. The above starting substances are thoroughly mixed and baked to form a detecting element. The element may be used as a lump by itself, but it is convenient that it is used as a film of the element baked on a heat-resisting insulating material such as an alumina ceramics, quartz glass or boracic silica glass.