To lower humidity dependence and to detect low concn. gas by applying and forming a coating layer containing a metal oxide combustion catalyst on a responsive layer based on indium oxide.

A fine powder of indium hydroxide is impregnated with an aq. soln. containing tin chloride in predetermined concn. to be dried and baked to obtain indium oxide which is, in turn, ground to be formed into paste. A platinum wire coil 1 is coated with this paste and, after drying, a current is allowed to flow to the coated coil 1 to bake the same to obtain a responsive layer 2. An ammonia aq. soln. is dripped on a mixed aq. soln. wherein tin chloride and cobalt nitrate are dissolved in predetermined concn. to obtain a precipitate and, after various ions of the precipitate are washed off, the precipitate is dried to obtain a stannic acid gel which is, in turn ground and baked to obtain tin oxide containing 0.5 atm.% of cobalt. oxide. This tin oxide is ground to be formed into paste and the responsive layer 2 is coated with the paste to form a catalyst layer 3. That is, the catalyst layer is made hard to receive the effect of humidity by using indium oxide and gas to be detected can be detected with high sensitivity and hydrocarbon gas can be detected with high selectivity by applying and forming the catalyst layer 3.