The principal aim of this study was to clarify the rationale of guaranteeof free speech and to examine a range of the guarantee in public libraries.Understanding the relationship between public libraries and free speechhelps to solve the subject. In conclusion, the present study suggested thatthe relationship between public libraries and free speech depends on thenature of object.The public forum doctrine, developed as a framework for protectedagainst government suppression on expression in the United States, can beapplied to public libraries. If we consider public library as (limited) publicforum, library patrons’ freedom of expression is guaranteed, but guaranteeis limited to this range. The reason is that the library cannot directlyguarantee the authors’ free speech due to materials selection.Approach focusing on professional duty is useful when consideringguarantee of author’s free speech. In this approach, the freedom ofexpression is interpreted as comprised in professional duty, therefore,librarians’ autonomy and control will be realized.