In a chlorine ion-selective electrode for analysis of chlorine ion in a liquid sample which comprises a silver metal layer, a silver chloride layer and a chlorine ion-selective membrane, superposed in order, the chlorine ion-selective membrane containing a hydrophobic organic polymer as a binder, the improvement wherein the hydrophobic organic polymer is a vinyl acetal polymer such as vinyl butyral or a copolymer of a vinyl acetal and at least one monomer copolymerizable therewith such as a copolymer of vinyl butyral and vinyl alcohol.