PURPOSE: To obtain a sensor wherein the evaporating amount of electrolyte is less and the output voltage is stable for a long period with respect to the elapse of years by plating metal on the outer wall of a synthetic resin container, and applying an epoxy-resin coating.

CONSTITUTION: A metal 16 is plated on the entire surface of the outer wall of a plastic container 1. An epoxy resin 17 is coated on the entire surface of the outer wall of the container 1. A partitioning wall film 4 which is a positive catalyst electrode, a lead negative electrode, electrolyte 11 and the like are contained in the container 1. When oxygen molecules in air are adsorbed in a carbon collecting electrode 5 through porous Teflon 3 and the partitioning wall 4, electromotive force is generated across the a positive electrode lead wire 7 and a negative-electrode lead wire 9. The evaporating property of water contained in the electrolyte 11 is high. The water tends to diffuse and evaporate into the outside through the wall of the container 1. Since the metal 16 is plated on the entire surface of the container and the epoxy resin 17 is coated, however, the evaporating amount of the electrolyte 11 which contributes to the corroding reaction phenomena of the partitioning wall 4 and the lead 8 can be made small.