The ABA-type triblock copolymers consisting of poly(2-adamantyl vinyl ether) [poly(2-AdVE)] as outer hard segments and poly(vinyl ether) having polar oxyethylene side chains [poly(MOEO1VE), poly(MOEO2VE), poly(MOEO3VE), or poly(MOEO4VE)] as inner soft segments were synthesized by sequential living cationic polymerization. The triblock copolymer compositions were similar to those of the feed ratios, and [2-AdVE]/[MOEOnVE]/[2-AdVE] ratios of copolymers were all approximately 150/700/150. The obtained triblock copolymers formed microphase-separated structure, and the free-standing membranes were obtained by solution-casting. The CO2 permeability coefficient (PCO2) of triblock copolymer, poly(2-AdVE)-b-poly(MOEO1VE)-b-poly(2-AdVE), was 113 barrer, which is much larger than that of poly(AdVE) [PCO2 = 25.2 barrer]. The permeability increased with increasing the content of the oxyethylene units in the triblock copolymers because the flexibility of the oxyethylene chains enhances gas diffusion in the polymer matrix. The triblock copolymers exhibited high CO2 permselectivity due to the good affinity of oxyethylene units to CO2, and their PCO2/PN2 were as large as 43.2-57.4.