TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Battery : In the battery, one enzyme, called enzyme 1 is an oxidation enzyme. The decomposition reaction carried out by enzyme 1 is an oxidation reaction and the electron formed in the oxidation is transferred to a coenzyme. When the coenzyme participates in the battery system, the enzyme activity of the oxidase carrying out the oxidation of the coenzyme is not less than the sum of the enzymatic activities of the enzymes participating in the formation of the reduced form of the coenzyme. The oxidized form of the coenzyme is NAD+ (nicotinamide adenine dinucleotide) and the reduced form of the coenzyme is NADH (reduced nicotinamide adenine dinucleotide). The enzyme participating in the formation of the oxidized form of the coenzyme is diahorase. In the fuel battery, electrons are transferred from the coenzyme to an electron mediator, which is vitamin K3. The fuel is at least one substance selected from alcohols, hydrocarbonates, lipids, proteins and organic acids, preferably CH3OH, C2H5OF and/or glucose. When the fuel is CH3OH, the enzymes which are degraded stepwise are an alcohol dehydrogenase, a formaldehyde dehydrogenase and a formic acid dehydrogenase. When the fuel is glucose, the enzymes which are degraded stepwise are a glucose dehydrogenase, a gluconokinase and a phosphogluconate dehydrogenase. When the fuel is CH3OH, an alcohol dehydrogenase (ADH), a formaldehyde dehydrogenase (FalDH) and a formic acid dehydrogenase (FateDH), as the enzymes degraded stepwise, and a diahorase (DI), as the enzyme participating in the formation of the oxidized form of the coenzyme, are used in the fuel battery, and the activities of the enzymes satisfy relationships (1) and (2).0 less than U(ADH) at most U(FalDH) at most U(FateDH) (1)U(ADH) + U(FalDH) + U(FateDH) at most U(DI) (2)U(ADH) = activity of alcohol dehydrogenase ;U(FalDH) = activity of formaldehyde dehydrogenase ;U(FateDH) = formic acid dehydrogenase ;U(DI) = activity of diahorase EXAMPLE - NAD+ (nicotinamide adenine dinucleotide) (5 mM) and CH3OH (1M) were added to a 0.1 M tris HCl buffer solution (3 ml). This solution was degassed by stirring and purging with Ar. Alcohol dehydrogenase (ADH) (25 units). Formaldehyde dehydrogenase (FalDH) (50 units) and formic acid dehydrogenase (FateDH) (75 units) were added to the solution while measuring the concentration of NADH (reduced nicotinamide adenine dinucleotide) from the observed optical density at 340 nm using a UV/visible spectrometer. The concentration of NADH was increased.