Disclosed herein is a method of separating carbon dioxide from a gas mixture using a biopolymer, including: introducing hemocyte or extrapallial fluid, extracted from shells, into a reactor filled with 1 L of a solution containing a calcium source, and then dissolving the hemocyte or extrapallial fluid in the solution to form a mixed solution; and mixing carbon dioxide or a gas mixture containing carbon dioxide into the mixed solution to separate carbon dioxide from the gas mixture.