Disclosed are a method and an apparatus for treating an effluent containing ammonia in which method and apparatus N.sub.2O concentration in the gas at the outlet of a catalyst tower does not rise to a high level even when the NH.sub.3 concentration in the effluent was reduced and the amount of hazardous substances formed is small; in the method and apparatus, an NH.sub.3-containing effluent A and a carrier gas (steam C and combustion gas F) are contacted in stripping tower 7 to transfer the NH.sub.3 from the NH.sub.3-containing effluent to a gas phase, the gas containing the generated NH.sub.3 is heated with pre-heater 19 and then contacted with catalyst layer 13 placed in catalyst tower 12 to decompose the NH.sub.3 into nitrogen and water; and at that time, the oxygen concentration in the gas to be introduced into catalyst tower 12 and the N.sub.2O concentration in the gas discharged from catalyst tower 12 are determined by measuring instruments 21 and 22, respectively, and the oxygen concentration in the gas to be introduced into catalyst tower 12 is adjusted by adjusting valve 17 so that the N.sub.2O concentration becomes within a prescribed range.