A carbon dioxide separator includes an absorption tower for producing a carbon dioxide-rich absorbent and a carbon dioxide-depleted flue gas by reaction of a carbon dioxide-containing flue gas and an absorbent contained therein; a regeneration tower for removing the carbon dioxide-rich absorbent transferred from the absorption tower in the presence of the flowing gas to separate the same into a carbon dioxide-rich treatment gas and a carbon dioxide-lean absorbent; and a separation membrane module for selectively membrane-separating and concentrating the carbon dioxide, wherein the carbon dioxide-containing flue gas is transferred to the absorption tower as a carbon dioxide-lean flue gas obtained via the separation membrane module, and the flowing gas is transferred to the regeneration tower as the carbon dioxide-rich flue gas obtained via the separation membrane module from the carbon dioxide-containing flue gas.