A gas sensor of SnO2-based materials has been made by thick-film technology utilizing hydrophobic silica as a binder. The technology can achieve a high productivity as well as a reduced humidity dependence and a sufficient mechanical strength of sensors. A thick-film sensor of SnO2incorporated with ThO2shows highly selective detection for CO gas, separated from H2gas; i.e., sensitivity to CO is 42 times higher than to H2at both gas concentrations of 50 ppm. An excellent humidity-independent sensitivity is also achieved.