To provide a method and a system for treating gas, by which carbon dioxide is removed and hydrogen is produced efficiently and stably at a low cost.

The system for treating the gas is provided with a gas treatment apparatus 1 which is provided with an electrolyte 10 through which sodium ions are permeated, an anode 11 which is arranged in the electrolyte 10 and to which an aqueous solution of a sodium compound is supplied, and a cathode 12 which is arranged in the electrolyte 10 and to which the gas to be treated where water or a sodium hydroxide aqueous solution is mixed, is supplied, and separates/removes the carbon dioxide in the gas as a carbonate and produces hydrogen gas on the cathode 12 side by applying DC voltage between the cathode 12 and the anode 11, and the system is provided with a means 14 for injecting water into a supplying line of the gas to be treated in the apparatus 1 by means of carrier gas, solubilizing the carbonate produced on the cathode 12, and discharging the solubilized carbonate to the outside of this system. When the gas to be treated is supplied to the apparatus 1, the sodium hydroxide aqueous solution can be injected together with water.