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Rough set theory is a computational model of approximate reasoning proposed by Z. Pawlak in 1982, which captures a given concept as two approximations represented by measurements. Pawlak reached this idea from discussions with medical doctors that measurements given by human beings may not define correctly a concept given by nature. Especially, he focused on the characteristics of medical diagnostic reasoning: one is a set of manifestations, say L, which is sufficient to diagnose flu. The other one is a set of symptoms observed in patients of flu, say U. From the viewpoint of set of objects, a set of patients belonging to flu, denoted by X, has a subset which satisfies L. Also, this set, X, is covered by U. Pawlak called Land U a lower approximation and upper approximation of X, both which give approximations of the concept flu. In other words, a concept can be captured by lower and upper approximaiton of …