Abstract: An electrochemical sensor is formed having a working electrode for detecting hydrogen peroxide surrounded by a cylinder portion, and with an enzymecontaining membrane at its tip. The membrane has a porous layer permeable to hydrogen peroxide between a layer containing an immobilized enzyme capable of decomposing hydrogen peroxide and a layer containing an immobilized enzyme capable of decomposing a substrate to form hydrogen peroxide. The cylinder portion is embedded in the layer containing the hydrogen peroxide decomposing enzyme and surrounds the working electrode such that the electrode is in contact with the porous layer but is not in contact with the layer containing the hydrogen peroxide decomposing enzyme. The layer containing the hydrogen peroxide forming enzyme is on a side of the porous layer opposite the electrode so as not to contact the electrode. Activity of the hydrogen peroxide decomposing enzyme is no more than one-fourth of the activity of the hydrogen peroxide forming enzyme.

Abstract: A recess of a predetermined thickness is formed in the bottom of a cylindrical vessel and a hole for the liquid-junction is formed at the center of the recess, to which is joined a resilient NH.sub.4.sup.+ selective membrane. Since the NH.sub.4.sup.+ selective membrane has a thickness larger by about 0.1 mm than the depth of the recess, the NH.sub.4.sup.+ selective membrane is protruded out of the bottom of the vessel by so much. A polyester woven fabric on which urease is immobilized is attached to the vessel so as to cover the NH.sub.4.sup.+ selective membrane. Since the immobilized enzyme membrane is applied with an appropriate tension by the slightly protruded ion selective membrane, bondability between the two membranes can be improved.