PURPOSE: To prepare the titled electrode capable of being stably used over a long period of time and excellent in durability, by subjecting an electrode having a signal wire connected thereto and a chloride ion exchange membrane formed to the surface thereof to heat treatment.

CONSTITUTION: For example, a PVC powder is dissolved in tetrahydrofran and a chloride ion sensitive substance and, according to necessity, a plasticizer are further mixed therein to obtain a solution which is, in turn, used in forming a chloride ion exchange membrane 5. That is, to one end part of an insulating pipe 1, an electrode 3 comprising a metal or conductive resin having a signal wire 2 connected thereto through said insulating pipe 1 is provided and the aforementioned solution is applied to the electrode 3 preliminarily coated with a binder 4 such as a silane coupling agent to form a membrane 5. Subsequently, heat treatment is carried out after natural drying to increase the physical strength of the membrane 5 and to eliminate stickiness on the surface thereof.