An ion sensor having an ion selectivity, which comprises an internal electrode of metal/metal salt, composed of an electroconductive layer of at least one metal and a layer of an insoluble salt of the metal in contact with the electroconductive layer, an ion selective membrane whose supporting membrane is composed of a hydrophobic polymer, and an intermediate layer capable of keeping water molecules, provided between the internal solid electrode and the ion selective membrane, where the organic compound having a water-keeping property is polymethylene glycol, polyethylene glycol or polypropylene glycol, each having a molecular weight of 200 to 600, and the inorganic compound having a water-keeping property is calcium chloride, gold chloride, magnesium perchlorate, magnesium fluoride or vanadium chloride dioxide, is suitable for analysis of specific ion species in a biological fluid with practically prolonged maintenance of properties of electrode.