Use of nickel or cobalt oxide as auxiliary combustion agent is being recommended for nitrogen determination by the micro-Dumas method. Hydrous manganese dioxide, prepared from potassium permanganate and manganese sulfate, liberates a large amount of pure oxygen by pyrolysis. In this method, a sample is weighed into platinum boatcovered with ca. 50mg. of manganese dioxide, and the air that comes into the combustion tube at the time of insertion of platinum boat is expelled in 15 seconds by half-way flash-back system, without receiving hot air. The boat is moved to the position with quartz magnetic boat-carrier, two azotometers are connected to the combustion tube, and combustion is effected by the double procedure, shortening the time required. Movable furnace reaches the temperature of 950° after 10 minutes, when electricity is cut off, and N2 gas is expelled 15 minutes later. By continuous operation, 3-4 runs per hour are possible.