Quite new chitin producing method from crab shell using sub-critical aqueous acetic acid solution was developed not only to produce chitin with the high purity, but also to decrease the initial and running costs in comparison with the conventional method. When the sub-critical water in high temperature of 533 – 593 K was used as a reaction solvent, the protein, pigments and lipids of crab shell were decomposed rapidly and completely. However, because calcium carbonate were not removed, the additional treatment by aqueous HCI solution was required for the production of high purity chitin. On the contrary, by using the sub-critical aqueous acetic acid solution (stoichiometric molar ratio of acetic acid is 3:1 for calcium carbonate) as a reaction solvent, it was possible to produce about 95 % purity chitin with white color under the conditions of relatively low temperature of 453 – 493 K and very short reaction time of 1 minute. This chitin was equivalent in purity to the commercial reagent produced from the crab shell with the conventional method. Furthermore, almost 100% purity chitin was obtained in the additional sub-critical treatment of 2 – 10 minutes. Since the decalcification, delipidation, decolorization and de-protein of crab shell were occurred simply, completely and simultaneously in this new chitin producing method, it was proven that the very high purity chitin could be produced in one stage process, short time treatment, and remarkably low cost in process without using strong acid, strong base and large waste water treatment for the neutralization, in comparison with the conventional method.