Siloxane compounds are removed from the atmospheres by silica supporting an organic sulfonic acid compound. The silica with the organic sulfonic acid compound has a specific surface area down to 500 m2/g and up to 750 m2/g and a pore volume down to 0.8 m3/g and up to 1.2 m3/g, both measured by nitrogen gas adsorption method and has a pore diameter down to 4 nm and up to 8 nm, at the peak of differential pore volume measured by nitrogen gas adsorption method. The durability of gas sensing element against siloxanes is improved.