A treatment process of a gas containing zero-valent mercury and a mercury separation system, by which the amount of an iodine compound used can be reduced when the zero-valent mercury is separated from the gas containing the zero-valent mercury by using the iodine compound. The process has a step of oxidizing the zero-valent mercury contained in the gas with a first liquid phase containing an alkali metal iodide, thereby obtaining a second liquid phase containing a divalent mercury ion and an iodide ion; a step of separating the divalent mercury ion as mercury sulfide by adjusting the pH of the second liquid phase; and a step of circulating a third liquid phase which is obtained by separating the mercury sulfide in the mercury separation step to use the third liquid phase as the first liquid phase in the mercury oxidation step.