The present invention aims to simplify the structure and a fabrication method of an ion-selective electrode in an ion concentration measuring device that measures an anion, particularly a chloride ion, in a biological component. To this end, in a potential difference measuring unit, a quaternary ammonium salt derivative serving as a ligand for an anion is immobilized to the surface of a gold electrode by using as a linker an insulative molecule forming a self-assembled monolayer. The potential difference measuring unit measures an electromotive force generated with anion binding, as an interface potential change on the surface of the gold electrode. In order to reduce the influence of adsorption of impurities on the electrode surface, a high-molecular weight polymer is physically adsorbed on the gold electrode and thus used when a biological component is measured.