PURPOSE: To obtain excellent preserving life and operational life and to make it possible to maintain excellent response, by forming an intermediate layer between an insulating material covering a part of a conductive electrode and an ion sensitive film which is formed on a part that is not covered with the insulating material.

CONSTITUTION: An intermediate layer 4 is formed between an insulating material (polyvinyl chloride) 3 covering a part of a conductive electrode (silver electrode whose surface is covered with silver chloride) 2 and an ion sensitive film (comprising polyvinyl chloride, dioctyl adipate, tetrahydrofuran and the like) 5. Thus, the contact part between the insulating material 3 and the sensitive film 5 is made small at the intermediate layer incorporating electrolyte (potassium chloride and the like). Therefore movement of additive in the sensitive film 5 into the insulating material 3 is suppressed, and deterioration is prevented. As a material for the intermediate layer 4, a high- hydrophilic polymer such as polyacryl acid and the like and their derivatives whose affinity to the additive in the sensitive film 5 is poor is suitable. In order that adhesion between the sensitive film 5 and the electrode 2 is not deteriorated, a part of the insulating film 3 is formed so that the sensitive film 5 directly comes into contact with the insulating material 3.