Abstract: An enzyme electrode composed of a carrier, an enzyme immobilized on a parr of the surface of the carrier, a coating film consisting of a thin film coating the portion where the enzyme is immobilized and being permeable for the substrate for the enzyme and an internal electrode capable of applying voltage to the surface where the enzyme is immobilized, said carrier being impregnated with a substance which acts as an electron transfer mediator, which is useful for measurement of concentration of the substrate in vital samples such as blood serum, blood plasma, urine etc.