PURPOSE: To make it possible to prevent peeling of an enzyme-fixed film by chemically bonding polymer or prepolymer contained in both the enzyme-fixed film and an ion sensitive film with a cross-linking agent.

CONSTITUTION: In a sensor, electrodes having sensitive films wherein an ion sensitive film 6 and an enzyme-fixed film 7 are sequentially laminated are used, and the sensitive value of specimen liquid is electrically detected. At this time, polymer or prepolymer having reactivity for cross-linking agent is contained in the reactive film 6 and the fixed film 7. The polymer or the prepolymer in both films 6 and 7 are chemically bonded with the cross-linking agent. The sensitive film 6 is the pH sensitive film containing polyvinyl-chloride- based polymer having a hydroxyl group. The fixed film 7 is the polymer and enzyme holding film having polymerizing double bonds. The cross-linking agent is the silane compound having the reactivity with the hydroxyl group and the polymerizing double bonds, respectively. The electrodes wherein the sensitive film 6 and the fixed film 7 are sequentially laminated have the laminated structure comprising upper layers 3a and 3b wherein silver chloride is the main component and lower layers 2a and 2b wherein silver is the main component.