An ion sensor and an ion sensor plate provided with an ion sensitive membrane which is characterized in that the ion sensitive membrane contains a material constituting the dissociation equilibrium of Ag/X, and an excessive quantity of an ion sensitive material or an anion scavenger. A coated film of surfactant is provided at the inner face of the passageway communicating with the test liquid-measuring electrode so as to add the surfactant to the test liquid. The plasticizer is added at a ratio of 65 to 80% by weight. The atomic ratio of silver and halogen in the silver halide layer is set to at least 1.2. The particle diameter of the modified silver halide layer is set to at most 1 &mgr;m.