Oxidation characteristics of nitrogenous compounds in supercritical water in a commercial-scale downflow type reactor were investigated using a simulant of industrial waste containing high concentrations of nitrogenous compound and inorganic salts. N,N-Dimethylacetamide was used as the model of nitrogenous compounds in industrial waste. Increase of reaction temperature or air ratio was effective for decreasing concentrations of ammonium and cyanide ions, which were the main residual nitrogen compounds in effluent, under the following reaction conditions: temperature 642 degrees C, pressure 23.44 MPa, air ratio 1.2, and waste flow rate 6.5 kg/min (residence time about 10 sec). The effect of air ratio was particularly remarkable, and all the analysis data (TOC, NH4+, CN-, NO2-, and NO3-) fell below the detection limit at the temperature of 654 degrees C and air ratio of 1.3. It was also found that an optimum waste flow rate (5.1 kg/min) exists that minimizes the concentration of residual nitrogen compounds in liquid effluent. Ammonium and cyanide ions are the main residual nitrogen compounds in liquid effluent at higher waste flow rate (6.5 kg/min), and nitrite and nitrate ions are the main ones at lower waste flow rate (4.1 kg/min). Furthermore, it was found that presence of inorganic salts in wastewater promoted the decomposition of ammonium and cyanide ions.