Nitrous oxide-containing gas is subjected to heat exchange with decomposed gas from a nitrous oxide decomposition catalyst-filled reactor and then contacted with a heater comprising integrally formed heating unit and baffles, wherein gaps are formed between the baffle-integrated heater and the unit body in order to alleviate the pressure difference in the gas flow channel, and subsequently introduced into a nitrous oxide catalyst-filled catalyst layer for decomposition of the nitrous oxide into nitrogen and oxygen. The nitrous oxide-containing gas is neutralized by continuous treatment.