The invention provides a N2O removal method for a waste gas which is capable of stably decomposing and removing N2O even if the steam concentration in the waste gas is fluctuated. N2O is reduced and removed by bringing the waste gas containing N2O into contact with a reducing agent in the presence of a N2O decomposition catalyst. The temperature of the waste gas to be brought into contact with the N2O decomposition catalyst or the addition amount of the reducing agent is controlled in accordance with the steam concentration in the waste gas in a prior stage of the N2O decomposition catalyst. Therefore, the N2O decomposition efficiency is prevented from being decreased. An iron-zeolite based catalyst may be used as the N2O decomposition catalyst and methane, propane, ammonia, etc. may be used as the reducing agent.