To approximately completely remove bromine gas in exhaust gas from a reactor even when concn. of bromine gas in the exhaust gas from the reactor for performing reaction of ammonia nitrogen with ozone becomes high in the case where ammonia nitrogen in an ammonia nitrogen-contg. drainage is removed by adding ozone into the ammonia nitrogen-contg. drainage in the presence of bromine ion.

By bringing exhaust gas exhausted from a reactor wherein ammonia nitrogen is reacted with ozone into contact with alkali soln., bromine gas included in the exhaust gas is removed. For example, the ammonia nitrogen- contg. drainage is treated in a reactor 2 and the exhaust gas exhausted from the reactor 2 is introduced into a packed tower type gas-liq. contact tank 30 through an exhaust gas exhaust conduit 16 and the exhaust gas is brought into contact with the alkali soln. in this gas-liq. contact tank 30 by gas-liq. counter current system. The alkali waste liq. 48 after bringing it into contact with the exhaust gas can be used as a pH adjusting agent for the ammonia nitrogen-contg. drainage.