NOVELTY - A fuel cell, comprises a structure with positive electrode and negative electrode facing each other through the proton conductor, where the negative electrode consists of a diaphorase and an electronic mediator and where reduced type NADH solution and/or reduced-type NADPH solution is used as a fuel. The concentration of reduced type NADH solution is 2 mM or more and a change in the enzyme reaction by diaphorase with respect to the concentration of the reduced-type NADH solution is represented by a curve with peak.    USE - The fuel is useful in the manufacture of an electronic device, electric power generation system, enzyme reaction utilization apparatus e.g. biosensor and a vehicle (claimed) such as motor vehicle, aircraft, rocket, spacecraft etc.    ADVANTAGE - The fuel cell generates electric power at high efficiency using solution containing NADH or NADPH as fuel and an enzyme as a catalyst. The fuel can be recycled and the cell enables manufacture of efficient electronic device, vehicle, electric power generation system etc.    DETAILED DESCRIPTION - A fuel cell, comprises a structure with positive electrode (2) and negative electrode (1) facing each other through the proton conductor (3), where the negative electrode consists of a diaphorase (13) and an electronic mediator (12) and where reduced type NADH solution and/or reduced-type NADPH solution is used as a fuel. The concentration of reduced type NADH solution is 2 mM or more and a change in the enzyme reaction by diaphorase with respect to the concentration of the reduced-type NADH solution is represented by a curve with peak, where the peak falls when the concentration of the reduced type NADH is less than 2 mM, and the concentration of solution containing reduced type NADH and/or reduced type NADPH is 2-130 mM.    INDEPENDENT CLAIMS are also included for:    (1) an electronic device using a fuel cell;    (2) a vehicle using the fuel cell;    (3) electric power generation system using the fuel cell; and    (4) enzyme reaction utilization apparatus, comprising the reduced type NADH and/or NADPH solution, diaphorase and electronic mediator.    DESCRIPTION OF DRAWING(S) - The figure shows the diagram of fuel cell. (Drawing includes non-English language text)    Negative electrode (1)    Positive electrode (2)    Proton conductor (3)    Electronic mediator (12)    Diaphorase (13)