To provide a gas purification system which can treat ammonia in gasified gas generated from organic waste or biomass fuels, and an ammonia treatment method in the gas purification system.

The gas purification system has gasification facilities 11 generating gasified gas from a solid fuel, and a wet type gas purification facilities A purifying the gasified gas by a washing means. The gas purification system is provided with a separation means 33 separating ammonia from washing waste water obtained from the wet type gas purification facilities A, and a supply means supplying the ammonia separated by the separation means to the gasification facilities 11.