A method of treating an emission gas, comprising bringing an emission gas containing at least sulfur dioxide and mercury into contact with an absorbent liquid to thereby remove the sulfur dioxide from the emission gas, wherein persulfuric acid is added to the absorbent liquid. Alternatively, an iodine gas is added to the emission gas, and the resultant gas is brought into contact with an absorbent liquid of wet flue gas desulfurization equipment. Accordingly, there can be provided a method of treating an emission gas in which even when fluctuations occur in power generation load and emission gas composition, a high rate of removal of sulfur dioxide and mercury can be stably maintained over a prolonged period of time.