In the study of library history in our country, the history of the Koryo Dynasty is very important. The Pisosong, the first library to be operated by the government, was founded in 995 .A. D. Although research about the Pisosong indicated that it was located outside the palace the author felt that further investigation was needed. Might not the library be located inside the palaceulcorner For this reason, this study examined 1) the historical background of the Pisosong, 2) changes in its structure and organization, 3) its organization and personnel, 4) its function and, 5) its relation to the Pigak and the Osowon. The findings of this study may be summarized as follows: 1) Pisosong was changed and reorganized 15 times by the end of the Koryo Dynasty. 2) Its functions were keeping official documents and books, buying, exchanging, publishing, editing, revising books, writing official documents for the king and, as a place to discuss ideas and sometimes to deliver lectures to the king. 3) Osowon was another institute which was equivalent in its function and role to the Pisosong. 4) The Pisosong was in all probability located inside the palace because there were very important books in it. It is assumed that its location should be a secure place. Also this would enable the library to be managed more conveniently. 5) The Pigak in the Koryo Dynasty must have been located inside the palace as the Piko in the Sung Dynasty in China was.