PURPOSE: To make the thickness of a layer uniform and to perform measurement of ion activity having reliability and reproducibility, by forming a function layer of an ion selective electrode in a cylindrical shape.

CONSTITUTION: A columnar member 11 is formed by a silver wire, platinum wire or copper wire, etc. plated with silver and an ion selective layer 14 is formed by an ion selective composition. The layer 14 is formed by immersing the member 11 in a coating solution containing the ion selective composition and having adjusted viscosity. When each sample solution to the inspected and a standard solution is added dropwise to each solution reservoir 38a, 38b, the tip end interfaces of both solutions come into contact with eah other at the middle of a bridge, and electrodes 10a, 10b are conducted. Thereupon, the potential difference indicated by a potentiometer 51 is read and the measurement of ion activity in the solution to be inspected is performed.