To harmlessly burn and dispose an ammonia-containing gas deaerated from a drain water of a water cleaning unit.

A method for treating the ammonia-containing gas comprises a step of converting a coal into a generating gas in a pressurized gasifying furnace 10, a step of dedusting the generated gas, feeding the dedusted gas via a heat exchanger 40 and then cooling and cleaning the gas by the water cleaning unit 50, a step of branching the generated gas required to be disposed at an abnormal time or the like and incinerating the gas in a generating gas incinerator 120, a step of removing H2S in the generated gas by an H2S removing unit 70 and then burning the gas in an H2S burning furnace 80, and a step of treating to burn the ammonia-containing gas deaerated at a pressure reducing time by a drain water of the unit 50, by one separate gas burning means of any of a plurality of the separate gas burning means having an auxiliary boiler, the furnace 80 and the incinerator 120.