Application of a bio-electro reactor process for the treatment of wastewater containing nitrate and internal source of hydrogen donor as organic matter was experimentally investigated. The bio-electro reactor consisted of immobilized denitrifying bacteria electrode as cathode and carbon electrode as anode. Hydrogen gas was produced on the cathode surface by the electrolysis of water when electric current was applied, and immediately utilized to reduce biologically nitrate to nitrogen gas by the cathodic immobilized denitrifying bacteria. Consumption characteristics of organic matter and utilization of hydrogen gas derived from the electrolysis of water for denitrification was studied by batch experiments. Continuous treatment experiments using a synthetic wastewater containing nitrate and organic matter were carried out to investigate the denitrification and organic matter removal performances. Denitrifcation occurred with simultaneous utilization of organic matter existed as internal source of hydrogen donor in the wastewater and hydrogen gas by electrolysis of water. Hydrogen gas was utilized efficiently for denitrification even if excess amount of organic hydrogen donor existed in wastewater. In this bio-electro reactor system, it was confirmed that COD as well as nitrate was removed simultaneously by applied electric current in continuous experiment, though further investigation are necessary to analyze the COD removal by applied electric current in detail.