A thin layer of a polymer containing densely populated redox-active nitroxide radicals, photocrosslinked poly[2,3-bis(2',2',6',6'-tetramethylpiperidin-1'-oxyl-4'-oxycarbonyl)-5-norbornene] (PTNB), was employed to demonstrate an electrochemical transistor-like behavior in the form of a polymer-sandwiched nanogap device. A significant and sensitive change in the current in response to the applied electrode potential was produced, owing to the fast formation of the steep redox gradient of the electroactive species through the polymer layer. A current response to an alternating voltage obtained from the polymer-sandwiched nanogap device exhibited the electrochemical amplification effect based on the facile redox conduction.