The National Diet Library (NDL) is the only national library in Japan. It was established in 1948 by the National Diet Library Law, based on Article 130 of the National Diet Law, "The National Diet Library shall be established in the Diet by a separate law, in order to assist Diet Members in their study and research".
The law declares the ideal of the library's establishment in its preamble: The National Diet Library is hereby established as a result of the firm conviction that truth makes us free and with the object of contributing to international peace and the democratization of Japan as promised in our Constitution.

The NDL's primary function is to assist Diet Members to perform their duties. At the same time, it has a mission to provide library services for the executive and judicial branches of the national government and for the general public. As the only depository library in Japan, the Library acquires all materials published in Japan, preserves them as national cultural heritage, compiles catalogs of these publications in a database or other format, and with these collections provides library services.