The filter of a gas sensor comprises an inorganic porous support supporting both an organic sulfonic acid compound including sulfo group (—SO3H) and a Lewis acid having at least a metal element of transitional metal elements, Al element, Ga element, In element, Ge element, and Sn element. The Lewis acid loaded in the inorganic porous support adsorbs low concentration siloxanes. The organic sulfonic acid compound including sulfo group polymerizes adsorbed siloxanes in the filter so as not to desorb from the filter.