Denitrification of nitrate-polluted water was investigated using a bio-electro reactor which consisted of immobilized denitrifying bacteria electrode as cathode and carbon electrode as anode. In this system, the denitrification occurred with the hydrogen produced by electrolysis of water ; CO₂ and H⁺ derived from electrochemical oxidation of anodic carbon acted as buffer, so, pH was maintained around neutral value. To enhance the denitrification capability of the cathodic biofilm, acetate was also added to the reactor. In the batch experiments, simultaneous use of acetate and hydrogen was observed. The denitrification rate increased with the increase of the concentration ratio between CH₃COO^--C mg·l^-1 and NO₃^--N mg·l^-1 which was given in the form of C/N ; however, acetate remained. The maximum denitrification rate without residual acetate was observed at C/N=1, which corresponds approximately to chemical stoichiometric ratio of the nitrate reduction reaction to nitrogen using acetate as hydrogen donor. In the case of 5 hours of HRT at the continuous experiment, nitrate reduction more than 90% occurred under C/N=1 without residual acetate and nitrite in the reactor effluent and the removal rata was 3.8 times as large as that of the condition without feeding acetate.