An ion-selective electrode comprises an ion-sensing membrane, an internal electrode adhered to the ion-sensing membrane, and a lead connected to said internal electrode. The internal electrode used comprises a plurality of metallic wires arranged into a grating or a mesh. The wires comprises a metal core and a surface layer of a salt of the metal. The metal of the core is silver and the salt of the metal is a silver halide selected from the group consisting of silver bromide, silver chloride and silver iodide. At least a part of the internal electrode is buried inside the ion-sensing membrane. Various components, the ion-sensing membrane, the internal electrode, the lead, an external terminal, are disposed in a flow-through cell having a flow path formed therein, through which path a sample solution is passed, to constitute the ion-selective electrode. In the flow-through cell is disposed a means of maintaining the humidity to feed water vapor therefrom to the ion-sensing membrane. This structure, even when the ion-sensing membrane has a complicated configuration such as concave or convex surface, enables the establishment of good adhering conditions between the internal electrode and the ion-sensing membrane while retaining the complicated configuration of the ion-sensing membrane without reducing the effective contact areas of the ion-sensing membrane and the internal electrode.