To efficiently treat a hydrazine-contg. waste water and to suppress by-product of ammonia nitrogen and nitrate nitrogen by a method wherein when ozone is incorporated in the hydrazine-contg. waste water and the mixture is reacted in a reactor to gasify the hydrazine into nitrogen, pH of water in the reactor is kept at a specified value.

A pH adjusting agent (an alkali agent) is injected into a hydrazine-contg. waste water introduced into an original water adjusting tank 2 from an original water inlet pipe 4 from a pH adjusting agent storing tank 16 by means of the first pH adjusting agent injecting pump to preliminarily adjust the pH of the waste water. Therefore, the waste water is introduced into the reactor 6 and on the other hand, an ozone-contg. gas generated in an ozone generator 10 is added to the water 38 in the tank from an aeration device 12 to gasify hydrazine in the waste water into nitrogen gas and the exhaust gas is exhausted from an exhaust pipe 36. In this time, pH of the water 38 in the tank is measured by means of a pH meter 26 and the amt. of a pH adjusting agent incorporated by the second pH adjusting agent injecting pump 24 is suppressed to keep pH of the water 38 in the tank at least 7.0.