It was established in 1948 for the purpose of assisting members of the National Diet of Japan (国会, Kokkai) in researching matters of public policy.

The original Diet libraries "never developed either the collections or the services which might have made them vital adjuncts of genuinely responsible legislative activity."

Until Japan's defeat, moreover, the executive had controlled all political documents, depriving the people and the Diet of access to vital information.

The U.S. occupation forces under General Douglas MacArthur deemed reform of the Diet library system to be an important part of the democratization of Japan after its defeat in World War II.

The National Diet Library opened in June 1948 in the present-day State Guest-House (former Akasaka Detached Palace) with an initial collection of 100,000 volumes.

In the fiscal year ending March 2004, for example, the library reported more than 250,000 reference inquiries; in contrast, it recorded only 32,000 requests for research from the National Diet.

Moreover, because the NDL serves as a research library for Diet members, their staff, and the general public, it maintains an extensive collection of materials published in foreign languages on a wide range of topics.

The NDL has an extensive microform collection of some 30 million pages of documents relating to the Occupation of Japan after World War II.

This collection include the documents prepared by General Headquarters (GHQ) and the Supreme Commander of the Allied Powers (SCAP), the Far Eastern Commission (FEC), and the United States Strategic Bombing Survey Team.

These materials include, among other things, foreign doctoral dissertations in the sciences, the proceedings and reports of academic societies, catalogues of technical standards, etc.

The major catalogues in this collection include: (1) some 6,000 documents relating to the Tokugawa shogunate (1603–1867), such as records of town magistrates, the shogunal Supreme Court, and the Jisha-bugyō (Commissioners of Shrines and Temples), as well as documents concerning the succession of shōguns; (2) the Itō Bunko and Shirai Bunko, consisting of 8,000 handwritten and woodblock printed books dating from the Edo and Meiji periods and concerning Japanese medicine; and (3) the Shinjo Bunko, consisting of 11,000 examples of pre-modern writings on astronomy and calendars, in addition to ancient Chinese works on the Qing dynasty, genealogy, and local history.

The NDL has transferred the following collections to the Kansai-kan: most western periodicals; books and other materials in non-Japanese Asian languages; certain scientific and technological materials (technical reports, papers of foreign academic societies, catalogs of Japanese and foreign technical standards, foreign doctoral dissertations, and conference proceedings in Western languages); scientific research reports compiled under grants from the Ministry of Education, Culture, Sports, Science and Technology; Japanese doctoral dissertations; and books on tape.

[16][17] One of the most important features of the NDL's website is the Digital Library of the Meiji Era (近代デジタルライブラリー, Kindai dejitaru raiburarii, lit.

The digital library contains actual images of these works, which are divided into ten main categories based on Nippon Decimal Classification (NDC) : (0) general (総記, sōki); (1) philosophy (哲学, tetsugaku); (2) history (歴史, rekishi); (3) social sciences (社会科学, shakai kagaku); (4) natural sciences (自然科学, shizen kagaku); (5) engineering and manufacturing (工学・工業, kōgaku/kōgyō); (6) industry (産業, sangyō); (7) arts and athletics (芸術・体育, geijutsu/taiiku); (8) language (語学, gogaku); and (9) literature (文学, bungaku).