To reduce installation cost or power cost, in an apparatus equipped with a nitration tank and a denitrification tank and circulating the nitration soln. of the nitration tank to the nitrification tank, by providing a denitrification soln. circulating passage containing a circulating pump and allowing nitration soln. to flow in the denitrification tank from the nitration tank by head difference.

The nitration soln. is allowed to flow in a denitrification tank 1 from a nitration tank 2 through a nitration soln. circulating inflow port 10 by head difference and org. matter in org. waste water is biologically oxidized and decomposed in an anaerobic atmosphere and nitrogen oxide in the nitration soln. is reduced to nitrogen gas to be discharged. A denitrification soln. is drawn out of the denitrification tank 1 by a circulation pump 3 through a denitrification soln. circulating passage 11 and mixed with oxygen-rich gas in an oxygen mixing device 4 and supplied to the nitration tank 2 from a liquid jet device 5 and the residual org. matter and a nitrogen compd. in raw water are oxidized and decomposed by aerobic bacteria in an aerobic atmosphere and a nitrogen compd. is oxidized to nitrogen oxide. The nitration soln. from which nitrogen is removed is separated into sedimented sludge and supernatant water in a sedimentation tank 7.