A gas sensor comprises a layered structure with an ionic conductive film and a high gas-permeability interlayer film, a first catalyst electrode and a second catalyst electrode, a conductivity promotion structure, a high-k layer and a current detecting unit. The ionic conductive film includes a material with ionic conductivity ranging from 0.02 to 1000 S/cm. The first catalyst electrode and second catalyst electrode are located on the layered structure and spaced by a predetermined distance for ionizing a gas and reducing the ionized gas, respectively. The conductivity promotion structure includes a material with electronic conductivity ranging from 10−5 to 105 S/cm, and provides wanted electrons for a reduction reaction. The high-k layer is interposed between the conductivity promotion structure and layered structure. The current detecting unit is coupled to the first catalyst electrode and second catalyst electrode to sense a detecting current with respect to the ionized gas.