This invention relates to a device for separating carbon dioxide that includes a self-recycling loop, and to a method of separating carbon dioxide, which serve to effectively separate carbon dioxide from a combustion gas using a separation membrane provided with the self-recycling loop. This invention adopts a self-recycling loop in which the residue gas passing through a specific separation membrane is introduced into another separation membrane and in which a permeate gas passing through the specific separation membrane is introduced back into the specific separation membrane. Accordingly, the concentration of carbon dioxide in the feed gas of the specific separation membrane is increased, which increases the concentration of the permeate gas to thus improve the separation performance of the separation membrane, thereby separating high-purity carbon dioxide.