To provide a gas sensor having a long life which does not generate carbonic acid gas.

Inside a case 1, for instance, an Au electrode 2 as a plus electrode and a Pb electrode 3 as a minus electrode are arranged so as to face each other, and an electrolyte 4 of acetic acid is filled between the electrodes 2 and 3. Lead acetate is formed from the acetic acid on the surface of the Pb electrode 3. When ozone 6 permeates a gas permeable film 5, the surface of the Pb electrodes 3 is oxidized to lead oxide. However, the lead oxide is dissolved immediately into the acetic acid electrolyte 4 thereby to regenerate the lead acetate surface.