The present invention relates to a PST-20 zeolite having a novel skeletal structure, its preparation method, and a selective separation and adsorption method for a gas using the PST-20 zeolite. More specifically, the present invention relates to a method of preparing a microporous aluminosilicate PST-20 zeolite having a novel skeletal structure totally different from the skeletal structure of known zeolites and using the PST-20 zeolite as an adsorbent/separator capable of selectively adsorbing/separating carbon dioxide to separate and collect carbon dioxide with high purity from burned gases or natural gases.