PURPOSE: To prevent the deformation of a diaphragm owing to the evaporation of the moisture in an electrolyte by mounting a plastic film into the vessel of a galvanic cell type oxygen sensor and forming a hermetic space between the film and the vessel wall.

CONSTITUTION: The galvanic cell type oxygen sensor is constituted by inserting and screwing a press plate 3, O-rings 4, 4', 4 the diaphragm 5 consisting of a film of a tetrafluoroethylene/hexafluoropropylene compolymer, a positive electrode 6 consisting of gold, carbon paper 7 having a function as an electrode holder and a function as a current collector and a positive electrode current collector 8 consisting of a titanium wire between a vessel body 1 and a cap 2. The electrolyte 9 consisting of an aq. soln. mixture composed of acetic acid, potassium acetate and lead acetate and the negative electrode 10 consisting of lead are contained in the body 1 and a space part 12 is formed between the PVC film 11 and the wall of the vessel 1. The decrease in the internal pressure of the sensor is suppressed and the deformation of the diaphragm is prevented even if the moisture in the electrolyte evaporates by constituting the sensor in the above- mentioned manner.