Gas sensor

The object is to provide a gas sensor having excellent humidity resistance even if used in a high temperature and high humidity atmosphere. Agas sensor (1) comprising: a silicon substrate (2) ; a metal-oxide semiconductor portion (41) comprised mainly of SnO 2 and formed on the substrate (2); and a catalytic portion (42) comprised of Pd and dispersed on a surface of the metal-oxide semiconductor portion (41), wherein the metal-oxide semiconductor portion (41) and the catalytic portion (42) constitute a gas sensing portion (4). Furthermore, an insulating portion (7) comprised mainly of SiO 2 is formed dispersedly on a surface of the gas sensing portion (4). Further, the catalytic portion (42) and the insulating portion (7) are formed on the surface of the metal-oxide semiconductor portion (41) so that the surface additive rate, which is expressed by Si/ (Pd+Si) - the ratio in the number of atoms of Si to Pd, of the gas sensing portion (4) having the insulating portion (7) may be 65% or more to 97% or less, and so that the surface additive rate, which is expressed by Si/(Sn+Si) - the ratio in the number of atoms of Si to Sn, of the gas sensing portion (4) may be 75% or more to 97% or less.