Denitrifying microorganisms were immobilized with a sodium alginate gel on a cathode electrode, and electric current was applied using a carbon electrode as the anode. Biological reductions of nitrate through the use of H2 at the cathode and formations of inorganic carbons at the anode were observed. Experimental results showed that oxidation of carbon electrode to CO2 was favorable for developing anoxic conditions and to neutralize alkalinity formed during denitrification. Several volts of potential was needed to operate the reactor.