The Library of Congress has dual functions in Japan. It is both the National Library of Japan and the Library of Congress. The National Library of Congress Law clearly points out that the purpose of the library is to help members of Congress complete their official duties, and at the same time provide library services for administrative and judicial departments and Japanese nationals as stipulated by law, thus stipulating that the service objects of the National Library of Congress are members of Congress, government agencies and ordinary citizens. Serving Congress occupies the first place, which is the focus of its service.

The National Library of Congress is the only library that legally accepts books submitted. Its collection of domestic publications will be preserved permanently as Japan's cultural heritage, so it also has the function of preserving libraries. In the information society, as a national document information center, it provides fast and accurate services for the needs from all aspects at home and abroad.