Being asked the following question:
"I'm a Professor of History of Book and Libraries. I'm looking for the name of architects of National Diet Library, of the Main Building and the Annexe Building. It's very important to us to know about the history of Japanese libraries architecture."
My reply as follows:
Thank you for your interest in Japanese architecture.
Generally both buildings are attributed to the design offices of Kunio Mayekawa (1905 - 1986), MAYEKAWA KUNIO ASSOCIATES, ARCHITECTS & ENGINEERS and a second design office with the name MIDO DOUJIN.
The official website of the succeeding office http://www.maekawa-assoc.co.jp/index-en.htm
To prevent any confusion with information from other sources, here is some more additional info.
(1) You may also encounter the romanized version Maekawa for his surname, without the 'y', which is more 'in line' with modern romanization systems, though the office used and still uses the 'y', if not in the website address, but in all the content of the site.
(2) Beside the official office name above you may also encounter other names like MAEKAWA KUNIO BUILDING DESIGN OFFICE, which is simply a word by word translation of the Japanese office name 前川国男建築設計事務所, romanized: Maekawa Kunio Kenchiku Sekkei Jimusho, though this is not used by the office itself.
(3) About MIDO DOUJIN, Japanese: ミド同人, romanized: Mido Doujin, this is a follow-up organization of the MAYEKAWA INSTITUTE OF DESIGN, Japanese: 前川設計研究所, romanized: Maekawa Sekkei Kenkyusho. More historical information on the many different offices can be found here (sorry Japanese only), www.jia.or.jp/topics/planning/2001/06_01-02.htm
(4) Regarding the National Diet Library Main Building, some official information from MLIT, the Japanese Ministry of Land, Infrastructure and Transport, http://www.mlit.go.jp/gobuild/gobuild_fr6_000032.html, also mentioning the staff designers in charge
-> design selected in a competition in 1953 (Showa 28)
-> designed by staff Makoto Tanaka and Masato Otaka of MAYEKAWA KUNIO ASSOCIATES, ARCHITECTS & ENGINEERS + 18 staff of MIDO DOUJIN
-> 1st phase completed in 1961 (Showa 36), 2nd phase in 1968 (Showa 43)
(5) Regarding the National Diet Library Annex, sometimes called the NDL New Building which is a direct translation of the Japanese: 国立国会図書館新館, romanized: Kokuritsu Kokkai Toshokan Shinkan, further literature sources of the time include the architecture magazine KENCHIKU BUNKA 1986.11 pp.21-42 and SHINKENCHIKU 1986.11 pp.195-206, https://rnavi.ndl.go.jp/research_guide/entry/theme-honbun-101098.php