NOVELTY - The biofuel cell comprises a polymer gel (2) reversibly swelling and contracting in response to variations in a property of a fuel solution (3) making contact with the polymer gels. The polymer gel is: placed on a surface of an electrode (1) and/or in the inside of the electrode; present in pores of the electrode and present in gaps between the carbon fibers; and a molecule-responsive gel, an ion-responsive gel, a proton-responsive gel and a temperature-responsive gel. The electrode has a porous material, and a laminate of carbon fibers.    USE - Used as a biofuel cell.    ADVANTAGE - The biofuel cell has high cell capacity, and is capable of effectively and automatically recovering lowered output due to fuel concentration gradient and pH gradient in a fuel solution by increasing the quantity of the fuel supplied to the electrode.    DETAILED DESCRIPTION - The biofuel cell comprises a polymer gel (2) reversibly swelling and contracting in response to variations in a property of a fuel solution (3) making contact with the polymer gels. The polymer gel is: placed on a surface of an electrode (1) and/or in the inside of the electrode; present in pores of the electrode and present in gaps between the carbon fibers; and a molecule-responsive gel, an ion-responsive gel, a proton-responsive gel and a temperature-responsive gel. The electrode has a porous material, and a laminate of carbon fibers. The variations in the property are variations in fuel concentration, ion concentration and temperature of the fuel solution. The molecule-responsive gel swells in the presence of a fuel and contracts in the absence of the fuel, and contracts in the presence of the fuel and swells in the absence of the fuel. The proton-responsive gel swells under a high-pH condition and contracts under a low-pH condition, and contracts under the high-pH condition and swells under the low-pH condition. The temperature-responsive gel contracts under a high-temperature condition and swells under a low-temperature condition, and swells under the high-temperature condition and contracts under the low-temperature condition.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a biofuel cell.    Electrode (1)    Polymer gel (2)    Fuel solution (3)    Biofuel cell. (A)