PURPOSE: To simplify the maintenance and operation and to stably reduce the nitrogen concn. by providing an oxidation equipment for supplying an oxidizing agent to the waste water contg. ammoniacal nitrogen and irradiating the water with UV to change the ammoniacal nitrogen into nitric nitrogen.

CONSTITUTION: When a waste water is sent to an oxidation raw water tank 1, the ammonium ion concn. and pH are measured by a first detector 2, and the waste water is then sent to an oxidation tank 4, mixed with hydrogen peroxide supplied from an oxidizing agent tank 6, agitated by an agitator 5A and oxidized. The waste water in the tank 4 is then sent to a UV oxidation tank 7, irradiated with UV to promote the oxidation reaction of the waste water, and the nitrogen in the waste water is practically nitrated to form nitrate ion. The waste water thus oxidized is then sent to a reduction raw water tank 8, the nitrate ion concn., pH of the water and ammonium ion concn. are measured by a second detector 9, an automatic switching valve 10B is switched in accordance with the result, and the water in the tank 8 is appropriately returned to the oxidation raw water tank 1.