Method for treating gas that contains zero-valent mercury, and mercury separation method

Provided are a method for treating gas that contains zero-valent mercury and a mercury separation system, with which the use of an iodine compound can be reduced when a zero-valent mercury is separated from gas that contains the zero-valent mercury. The method for treating gas that contains zero-valent mercury comprises: a mercury oxidation step for using a first liquid phase containing an alkali metal iodide to oxidize a zero-valent mercury contained in gas containing the zero-valent mercury, incorporating the oxidized mercury into the first liquid phase, thereby obtaining a second liquid phase comprising divalent mercury ions and iodide ions; a mercury separation step for using a pH adjuster to adjust the pH of the second liquid phase obtained in the mercury oxidation step, and adding an alkali metal sulfide, thereby separating the divalent mercury ions as mercury sulfide; and a first circulating step for circulating a third liquid phase containing the alkali metal ions and iodide ions obtained when the mercury sulfide was separated in the mercury separation step, the circulated third liquid phase being used as the first liquid phase of the mercury oxidation step.