To provide an exhaust gas treating system capable of inhibiting corrosion of a fly ash collecting device even if acidic gases are contained in an exhaust gas.

The exhaust gas treating system is provided with a primary cooling device 5 primarily cooling the exhaust gas from a furnace 3 accompanied by the generation of the exhaust gas, a secondary cooling device 7 secondarily cooling the exhaust gas after primary cooling by spraying cooling water and the fly ash dust-collecting device 6 collecting fly ash from the exhaust gas after primary cooling. After fly ash is collected by the fly ash dust-collecting device 6 from the exhaust gas after primary cooling, the exhaust gas after collecting fly ash is secondarily cooled by the secondary cooling device 7. After the exhaust gas is desalted after secondary cooling, a reaction product generated by desalting is separated and removed by a reaction product dust-collecting device 8 to be discharged outside of the system, or the exhaust gas is secondarily cooled by using an alkaline aqueous solution as the cooling water of the secondary cooling device and the reaction product generated by secondary cooling is separated and removed by the reaction product dust-collecting device to be discharged outside of the system.