Social tagging or collaborative tagging is a new trend in the organization, management, and discovery of digital information. Folksonomy, a collection of tags assigned to online information by all participants of social tagging systems, has many advantages as an information organization tool. As an information retrieval tool, however, folksonomy lacks the capability of discerning the relevancy of information. We propose a conceptual framework of IR for folksonomy on the basis of Library of Congress Subject Headings. Algorithms for constructing the conceptual framework are described, and values of the framework are also discussed.