A glucose sensor, using glucose oxidase immobilized in an electrically conductive polymer membrane, is developed for transient-mode measurement by on-off reaction control in the present work. An implantable glucose sensor loses its stability under the skin by fouling resulting from fibroblast adhesion on the glucose oxidase-immobilized membrane. Change in glucose distribution inside the membrane and in the vicinity of the surface of the membrane resulting from the fouling makes the measurement of the glucose concentration unstable. A technique of measuring transient current of the glucose sensor after activation of glucose oxidase may hardly be affected by the fouling as oxidation of coenzyme of glucose oxidase is rapid and the transient current is sensitive to glucose concentration. A glucose sensor with glucose oxidase immobilized in an electrically conductive polypyrrole membrane was fabricated. The activity of the glucose oxidase is switched on and off by a stepwise change of potential of the polypyrrole membrane from 0.15 V to 0.45 V versus a saturated calomel electrode. The two conditions described above can be confirmed by measuring transient current. Current increased momentarily after activation of glucose oxidase and decreased moderately after that. The oxidation and reduction of glucose oxidase is sufficiently rapid, and current intensity depends on glucose concentration in the test solution.