The National Diet Library is the successor of three separate institutions: the library of the House of Peers, the library of the House of Representatives, and the Imperial Library. The first two were established in 1890, when Japan created its Imperial Diet. The Imperial Library traced its roots to 1872, operating under the Ministry of Education. In pre-war Japan, the Diet's power was limited, and its libraries reflected that weakness -- they never developed the collections or services that might have supported genuinely independent legislative activity. The occupation authorities recognized that an informed legislature required an informed library. American library experts worked alongside Japanese reformers, but the occupation officers themselves reported that local initiative predated their arrival. The successful transformation owed much to dedicated Japanese leaders like Hani, who saw in the library not just a repository of knowledge but a tool for democratic revolution.

The library's first home was the Akasaka Detached Palace, now Japan's State Guest House -- an unlikely venue for a democratic institution, a former imperial residence repurposed for public knowledge. The first Librarian of the Diet Library was Tokujiro Kanamori, a politician. The philosopher Masakazu Nakai served as the first Vice Librarian. The collection quickly outgrew its palatial quarters. The library moved to its permanent building in Nagatachō, adjacent to the National Diet Building. By 1986, the NDL's Annex was completed to accommodate a combined total of 12 million books and periodicals. In 2002, the Kansai-kan opened in the Kansai Science City in Kyoto Prefecture, housing 6 million additional items. That same year, the International Library of Children's Literature opened in the former Imperial Library building in Ueno, holding 400,000 items of children's literature from around the world.

As Japan's legal deposit library, the NDL collects copies of every publication produced in the country. Its eight major specialized collections span an astonishing range. The Modern Political and Constitutional History Collection holds 300,000 items documenting Japan's legal modernization, including the personal archives of statesmen like Ito Hirobumi, Iwakura Tomomi, and Terauchi Masatake from the Meiji and Taisho periods. Some 30 million pages of microfilmed documents chronicle the postwar occupation, including records from General Headquarters and the Supreme Commander of the Allied Powers. The map collection holds 440,000 maps dating back to the early Meiji period. The music collection preserves 300,000 vinyl records and 200,000 compact discs -- every phonographic recording made in Japan. Among the rarest treasures are foreign-language accounts of Japan by European missionaries from the sixteenth century, and handwritten Edo-period manuscripts on medicine, astronomy, and shogunal law.

Though the NDL was created to serve the Diet, the general public long ago became its largest user. In the fiscal year ending March 2004, the library handled more than 250,000 reference inquiries from the public, compared to just 32,000 requests from the legislature it was built to support. The NDL has also embraced digital transformation. Its Digital Collections make rare books, audiovisual materials, and public-domain works available online. The Digital Library from the Meiji Era digitized some 60,000 volumes of Meiji-period publications, preserving them as searchable images. Japan Search, a web portal launched in 2020, draws on over 28 million items from 198 databases maintained by partner organizations across the country. Hani Goro's citadel of popular sovereignty has grown far beyond anything the imprisoned historian imagined -- a national memory bank accessible to anyone with a library card or an internet connection.